=========
NeuralNet
=========

Using NeuralNet
---------------

:class:`.NeuralNet` and the derived classes are the main touch point
for the user. They wrap the PyTorch :class:`~torch.nn.Module` while
providing an interface that should be familiar for sklearn users.

Define your :class:`~torch.nn.Module` the same way as you always do.
Then pass it to :class:`.NeuralNet`, in conjunction with a PyTorch
criterion.  Finally, you can call :func:`~skorch.net.NeuralNet.fit`
and :func:`~skorch.net.NeuralNet.predict`, as with an sklearn
estimator. The finished code could look something like this:

.. code:: python

    class MyModule(torch.nn.Module):
        ...

    net = NeuralNet(
        module=MyModule,
        criterion=torch.nn.NLLLoss,
    )
    net.fit(X, y)
    y_pred = net.predict(X_valid)

Let's see what skorch did for us here:

- wraps the PyTorch :class:`~torch.nn.Module` in an sklearn interface
- converts :class:`numpy.ndarray`\s to PyTorch
  :class:`~torch.Tensor`\s if necessary
- abstracts away the fit loop
- takes care of batching the data

You therefore have a lot less boilerplate code, letting you focus on
what matters. At the same time, skorch is very flexible and can be
extended with ease, getting out of your way as much as possible.

Initialization
^^^^^^^^^^^^^^

In general, when you instantiate the :class:`.NeuralNet` instance, the given
arguments are stored exactly as you pass them to :class:`.NeuralNet`. For
instance, the ``module`` will remain uninstantiated. This is to make sure that
the arguments you pass are not touched afterwards, which makes it possible to
clone the :class:`.NeuralNet` instance.

Only when the :func:`~skorch.net.NeuralNet.fit` or
:func:`~skorch.net.NeuralNet.initialize` method are called, are the different
attributes of the net, such as the ``module``, initialized. The name of an
initialized attribute always ends on an underscore; e.g., the initialized
``module`` is called ``module_`` (this is the same nomenclature as sklearn
uses). Thefore, you always know which attributes you set and which ones were
created by :class:`.NeuralNet`.

The only exception is the :ref:`history <history>` attribute, which is
not set by the user.

Most important arguments and methods
------------------------------------

A complete explanation of all arguments and methods of
:class:`.NeuralNet` are found in the skorch API documentation. Here we
focus on the main ones.

module
^^^^^^

This is where you pass your PyTorch :class:`~torch.nn.Module`.
Ideally, it should not be instantiated. Instead, the init arguments
for your module should be passed to :class:`.NeuralNet` with the
``module__`` prefix. E.g., if your module takes the arguments
``num_units`` and ``dropout``, the code would look like this:

.. code:: python

    class MyModule(torch.nn.Module):
        def __init__(self, num_units, dropout):
            ...

    net = NeuralNet(
        module=MyModule,
        module__num_units=100,
        module__dropout=0.5,
        criterion=torch.nn.NLLLoss,
    )

It is, however, also possible to pass an instantiated module, e.g. a
PyTorch :class:`~torch.nn.Sequential` instance.

Note that skorch does not automatically apply any nonlinearities to
the outputs (except internally when determining the PyTorch
:class:`~torch.nn.NLLLoss`, see below). That means that if you have a
classification task, you should make sure that the final output
nonlinearity is a softmax. Otherwise, when you call
:func:`~skorch.net.NeuralNet.predict_proba`, you won't get actual
probabilities.

criterion
^^^^^^^^^

This should be a PyTorch or PyTorch-compatible criterion.

When you use the :class:`.NeuralNetClassifier`, the criterion is set
to PyTorch :class:`~torch.nn.NLLLoss` by default. Furthermore, if you
don't change the loss to another criterion,
:class:`.NeuralNetClassifier` assumes that the module returns
probabilities and will automatically apply a logarithm on them (which
is what :class:`~torch.nn.NLLLoss` expects).

For :class:`.NeuralNetRegressor`, the default criterion is PyTorch
:class:`~torch.nn.MSELoss`.

After initializing the :class:`.NeuralNet`, the initialized criterion
will stored in the ``criterion_`` attribute.

optimizer
^^^^^^^^^

This should be a PyTorch optimizer, e.g. :class:`~torch.optim.SGD`.
After initializing the :class:`.NeuralNet`, the initialized optimizer
will stored in the ``optimizer_`` attribute.  During initialization
you can define param groups, for example to set different learning
rates for certain parameters. The parameters are selected by name with
support for wildcards (globbing):

.. code:: python

    optimizer__param_groups=[
        ('embedding.*', {'lr': 0.0}),
        ('linear0.bias', {'lr': 1}),
    ]

Your use case may require an optimizer whose signature differs from the
signature of a default PyTorch optimizer. In that case, you can define a custom
function that reroutes the arguments as needed and pass it to the ``optimizer``
parameter:

.. code:: python

    # custom optimizer to encapsulate Adam
    def make_lookahead(parameters, optimizer_cls, k, alpha, **kwargs):
        optimizer = optimizer_cls(parameters, **kwargs)
        return Lookahead(optimizer=optimizer, k=k, alpha=alpha)

    net = NeuralNetClassifier(
            ...,
            optimizer=make_lookahead,
            optimizer__optimizer_cls=torch.optim.Adam,
            optimizer__weight_decay=1e-2,
            optimizer__k=5,
            optimizer__alpha=0.5,
            lr=1e-3)

lr
^^^

The learning rate. This argument exists for convenience, since it
could also be set by ``optimizer__lr`` instead. However, it is used so
often that we provided this shortcut. If you set both ``lr`` and
``optimizer__lr``, the latter has precedence.

max_epochs
^^^^^^^^^^

The maximum number of epochs to train with each
:func:`~skorch.net.NeuralNet.fit` call. When you call
:func:`~skorch.net.NeuralNet.fit`, the net will train for this many
epochs, except if you interrupt training before the end (e.g. by using
an early stopping callback or interrupt manually with *ctrl+c*).

If you want to change the number of epochs to train, you can either
set a different value for ``max_epochs``, or you call
:func:`~skorch.net.NeuralNet.fit_loop` instead of
:func:`~skorch.net.NeuralNet.fit` and pass the desired number of
epochs explicitly:

.. code:: python

    net.fit_loop(X, y, epochs=20)


batch_size
^^^^^^^^^^

This argument controls the batch size for ``iterator_train`` and
``iterator_valid`` at the same time. ``batch_size=128`` is thus a
convenient shortcut for explicitly typing
``iterator_train__batch_size=128`` and
``iterator_valid__batch_size=128``. If you set all three arguments,
the latter two will have precedence.

train_split
^^^^^^^^^^^

This determines the :class:`.NeuralNet`\'s internal train/validation
split. By default, 20% of the incoming data is reserved for
validation. If you set this value to ``None``, all the data is used
for training and validation is skipped.

Note that there can be information leakage of the internal validation set into
the training set. E.g. if you use :class:`~skorch.callbacks.EarlyStopping`, by
default, it will monitor the validation loss and stop training if that loss
stops improving. It's therefore recommended to have yet another, external
validation set to perform independent validation.

Moreover, if you use :class:`~sklearn.model_selection.GridSearchCV` or something
similar, it already splits the training data into train and validation set, thus
you don't need skorch to do that (except if you use early stopping). In that
case, set ``train_split=None``.

For more details, please look at :ref:`dataset <dataset>`.

callbacks
^^^^^^^^^

For more details on the callback classes, please look at
:ref:`callbacks <skorch.callbacks>`.

By default, :class:`.NeuralNet` and its subclasses start with a couple
of useful callbacks. Those are defined in the
:func:`~skorch.net.NeuralNet.get_default_callbacks` method and
include, for instance, callbacks for measuring and printing model
performance.

In addition to the default callbacks, you may provide your own
callbacks. There are a couple of ways to pass callbacks to the
:class:`.NeuralNet` instance. The easiest way is to pass a list of all
your callbacks to the ``callbacks`` argument:

.. code:: python

    net = NeuralNet(
        module=MyModule,
        callbacks=[
            MyCallback1(...),
            MyCallback2(...),
        ],
    )

Inside the :class:`.NeuralNet` instance, each callback will receive a separate
name. Since we provide no name in the example above, the class name will taken,
so in the example above, the names would be ``"MyCallback_1"`` and
``"MyCallback_2"``. If you want to control the name of the callbacks, pass
tuples of strings and callback instances instead:

.. code:: python

    net = NeuralNet(
        module=MyModule,
        callbacks=[
            ('cb1', MyCallback1(...)),
            ('cb2', MyCallback2(...)),
        ],
    )

This approach of passing a list of *name*, *instance* tuples should be
familiar to users of sklearn\ :class:`~sklearn.pipeline.Pipeline`\s
and :class:`~sklearn.pipeline.FeatureUnion`\s.

One advantage of this way of passing callbacks is that it allows to pass
arguments to the callbacks by name (using the double-underscore notation):

.. code:: python

    net.set_params(callbacks__cb1__foo=123, callbacks__cb2__bar=456)

Use this, for instance, when trying out different callback parameters
in a grid search.

*Note*: The user-defined callbacks are always called in the same order
as they appeared in the list. If there are dependencies between the
callbacks, the user has to make sure that the order respects them.
Also note that the user-defined callbacks will be called *after* the
default callbacks so that they can make use of the things provided by
the default callbacks. The only exception is the default callback
:class:`~skorch.callbacks.PrintLog`, which is always called last.

warm_start
^^^^^^^^^^

This argument determines whether each
:func:`~skorch.net.NeuralNet.fit` call leads to a re-initialization of
the :class:`.NeuralNet` or not. By default, when calling
:func:`~skorch.net.NeuralNet.fit`, the parameters of the net are
initialized, so your previous training progress is lost (consistent
with the sklearn ``fit()`` calls). In contrast, with
``warm_start=True``, each :func:`~skorch.net.NeuralNet.fit` call will
continue from the most recent state.

device
^^^^^^

As the name suggests, this determines which computation device should
be used. If set to ``'cuda'``, the incoming data will be transferred to
CUDA before being passed to the PyTorch :class:`~torch.nn.Module`. The
device parameter adheres to the general syntax of the PyTorch device
parameter. If you want to prevent skorch from handling the device, set
``device=None``.

initialize()
^^^^^^^^^^^^

As mentioned earlier, upon instantiating the :class:`.NeuralNet`
instance, the net's components are not yet initialized. That means,
e.g., that the weights and biases of the layers are not yet set. This
only happens after the :func:`~skorch.net.NeuralNet.initialize` call.
However, when you call :func:`~skorch.net.NeuralNet.fit` and the net
is not yet initialized, :func:`~skorch.net.NeuralNet.initialize` is
called automatically. You thus rarely need to call it manually.

The :func:`~skorch.net.NeuralNet.initialize` method itself calls a
couple of other initialization methods that are specific to each
component. E.g., :func:`~skorch.net.NeuralNet.initialize_module` is
responsible for initializing the PyTorch module. Therefore, if you
have special needs for initializing the module, it is enough to
override :func:`~skorch.net.NeuralNet.initialize_module`, you don't
need to override the whole :func:`~skorch.net.NeuralNet.initialize`
method.

fit(X, y)
^^^^^^^^^

This is one of the main methods you will use. It contains everything
required to train the model, be it batching of the data, triggering
the callbacks, or handling the internal validation set.

In general, we assume there to be an ``X`` and a ``y``. If you have
more input data than just one array, it is possible for ``X`` to be a
list or dictionary of data (see :ref:`dataset <dataset>`). And if your
task does not have an actual ``y``, you may pass ``y=None``.

If you fit with a PyTorch :class:`~torch.utils.data.Dataset` and don't
explicitly pass ``y``, several components down the line might not work
anymore, since sklearn sometimes requires an explicit ``y`` (e.g. for
scoring). In general, PyTorch :class:`~torch.utils.data.Dataset`\s
should work, though.

In addition to :func:`~skorch.net.NeuralNet.fit`, there is also the
:func:`~skorch.net.NeuralNet.partial_fit` method, known from some
sklearn estimators. :func:`~skorch.net.NeuralNet.partial_fit` allows
you to continue training from your current status, even if you set
``warm_start=False``. A further use case for
:func:`~skorch.net.NeuralNet.partial_fit` is when your data does not
fit into memory and you thus need to have several training steps.

*Tip*:
skorch gracefully catches the :class:`KeyboardInterrupt`
exception. Therefore, during a training run, you can send a
``KeyboardInterrupt`` signal without the Python process exiting
(typically, ``KeyboardInterrupt`` can be triggered by *ctrl+c* or, in
a Jupyter notebook, by clicking *Kernel* -> *Interrupt*). This way, when
your model has reached a good score before ``max_epochs`` have been
reached, you can dynamically stop training.

predict(X) and predict_proba(X)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

These methods perform inference on the input data and return
:class:`numpy.ndarray`\s. By default,
:func:`~skorch.net.NeuralNet.predict_proba` will return whatever it is
that the ``module``\'s :func:`~torch.nn.Module.forward` method
returns, cast to a :class:`numpy.ndarray`. If
:func:`~torch.nn.Module.forward` returns multiple outputs as a tuple,
only the first output is used, the rest is discarded.

If the :func:`~torch.nn.Module.forward`\-output can not be cast to a
:class:`numpy.ndarray`, or if you need access to all outputs in the
multiple-outputs case, consider using either of
:func:`~skorch.net.NeuralNet.forward` or
:func:`~skorch.net.NeuralNet.forward_iter` methods to generate outputs
from the ``module``. Alternatively, you may directly call
``net.module_(X)``.

In case of :class:`.NeuralNetClassifier`, the
:func:`~skorch.classifier.NeuralNetClassifier.predict` method tries to
return the class labels by applying the argmax over the last axis of
the result of
:func:`~skorch.classifier.NeuralNetClassifier.predict_proba`.
Obviously, this only makes sense if
:func:`~skorch.classifier.NeuralNetClassifier.predict_proba` returns
class probabilities or logits. If this is not true, you should avoid 
:func:`~skorch.classifier.NeuralNetClassifier.predict` and just use
:func:`~skorch.classifier.NeuralNetClassifier.predict_proba` to retrieve predictions.

score(X, y)
^^^^^^^^^^^

This method returns the mean accuracy on the given data and labels for
classifiers and the coefficient of determination R^2 of the prediction for
regressors. :class:`.NeuralNet` has no score method. If you need it,
you have to implement it yourself.

model persistence
^^^^^^^^^^^^^^^^^

In general there are different ways of saving and loading models, each
with their own advantages and disadvantages. More details and usage
examples can be found here: :ref:`save_load`.

If you would like to use pickle (the default way when using
scikit-learn models), this is possible with skorch nets. This saves
the whole net including hyperparameters etc. The advantage is that you
can restore everything to exactly the state it was before. The
disadvantage is it's easier for code changes to break your old saves.

Additionally, it is possible to save and load specific attributes of
the net, such as the ``module``, ``optimizer``, or ``history``, by
calling :func:`~skorch.net.NeuralNet.save_params` and
:func:`~skorch.net.NeuralNet.load_params`. This is useful if you're
only interested in saving a particular part of your model, and is more
robust to library version changes.

Finally, it is also possible to use callbacks to save and load models,
e.g. :class:`.Checkpoint`. Those should be used if you need to have
your model saved or loaded at specific times, e.g. at the start or end
of the training process.

Input data
----------

Regular data
^^^^^^^^^^^^

skorch supports numerous input types for data. Regular input types
that should just work are numpy arrays, torch tensors, scipy sparse
CSR matrices, and pandas DataFrames (for the latter, see also
:class:`~skorch.helper.DataFrameTransformer`).

Typically, your task should involve an ``X`` and a ``y``. If you're
dealing with a task that doesn't require a target (say, training an
autoencoder), you can just pass ``y=None``. Make sure your loss
function deals with this appropriately.

Datasets
^^^^^^^^

:class:`~torch.utils.data.Dataset`\s are also supported, with the
requirement that they should return exactly two items (``X`` and
``y``). For more information on that, take a look at the
:ref:`Dataset` documentation.

Many PyTorch libraries, like torchvision, implement their own
``Dataset``\s. These usually work seamlessly with skorch, as long as
their ``__getitem__`` methods return two outputs. In case they don't,
consider overriding the ``__getitem__`` class and re-arranging the
ouputs so that ``__getitem__`` returns exactly two elements. If the
original implementation returns more than two elements, take a look at
the next section to get an idea how to deal with that.

Multiple input arguments
^^^^^^^^^^^^^^^^^^^^^^^^

In some cases, the input actually consists of multiple inputs. E.g.,
in a text classification task, you might have an array that contains
the integers representing the tokens for each sample, and another
array containing the number of tokens of each sample. skorch has you
covered here as well.

You could supply a list or tuple with all your inputs (``net.fit([tokens,
num_tokens], y)``), but we actually recommend another approach. The best
way is to pass the different arguments as a dictionary. Then the keys
of that dictionary have to correspond to the argument names of your
module's ``forward`` method. Below is an example:

.. code:: python

    X_dict = {'tokens': tokens, 'num_tokens': num_tokens}

    class MyModule(nn.Module):
        def forward(self, tokens, num_tokens):  # <- same names as in your dict
            ...

    net = NeuralNet(MyModule, ...)
    net.fit(X_dict, y)

As you can see, the ``forward`` method takes arguments with exactly
the same name as the keys in the dictionary. This is how the different
inputs are matched. To make this work with
:class:`~sklearn.model_selection.GridSearchCV`, please use
:class:`~skorch.helper.SliceDict`.

Using a dict should cover most use cases that involve multiple
inputs. However, it will fail if your inputs have different
sizes. E.g., if your array of tokens has 1000 elements but your array
of number of tokens has 2000 elements, this would fail. The main
reason for this is batching: How can we know which elements of the two
arrays belong in the same batch?

If your input consists of multiple inputs with different sizes, your
best bet is to implement your own dataset class. That class should
know how it deals with the different inputs, i.e. which elements
belong to the same sample. Again, please refer to the :ref:`Dataset`
section for more details.

Special arguments
-----------------

In addition to the arguments explicitly listed for
:class:`.NeuralNet`, there are some arguments with special prefixes,
as shown below:

.. code:: python

    class MyModule(torch.nn.Module):
        def __init__(self, num_units, dropout):
            ...

    net = NeuralNet(
        module=MyModule,
        module__num_units=100,
        module__dropout=0.5,
        criterion=torch.nn.NLLLoss,
        criterion__weight=weight,
        optimizer=torch.optim.SGD,
        optimizer__momentum=0.9,
    )

Those arguments are used to initialize your ``module``, ``criterion``,
etc. They are not fixed because we cannot know them in advance; in
fact, you can define any parameter for your ``module`` or other
components.
