Add Flax Dropout guide

docs/guides/dropout.rst

@@ -0,0 +1,299 @@
+Dropout
+=======
+
+This guide provides an overview of how to apply
+`dropout <https://jmlr.org/papers/volume15/srivastava14a/srivastava14a.pdf>`__
+using :meth:`flax.linen.Dropout`.
+
+Dropout is a stochastic regularization technique that randomly removes hidden
+and visible units in a network.
+
+Throughout the guide, you will be able to compare code examples with and without
+Flax ``Dropout``.
+
+.. testsetup::
+
+  import flax.linen as nn
+  import jax.numpy as jnp
+  import jax
+  import optax
+
+Split the PRNG key
+******************
+
+Since dropout is a random operation, it requires a pseudorandom number generator
+(PRNG) state. Flax uses JAX's (splittable) PRNG keys, which have a number of
+desirable properties for neutral networks. To learn more, refer to the
+`Pseudorandom numbers in JAX tutorial <https://jax.readthedocs.io/en/latest/jax-101/05-random-numbers.html>`__.
+
+**Note:** Recall that JAX has an explicit way of giving you PRNG keys:
+you can fork the main PRNG state (such as ``key = jax.random.PRNGKey(seed=0)``)
+into multiple new PRNG keys with ``key, subkey = jax.random.split(key)``. You
+can refresh your memory in
+`🔪 JAX - The Sharp Bits 🔪 Randomness and PRNG keys <https://jax.readthedocs.io/en/latest/notebooks/Common_Gotchas_in_JAX.html#jax-prng>`__.
+
+Begin by splitting the PRNG key using
+`jax.random.split() <https://jax.readthedocs.io/en/latest/_autosummary/jax.random.split.html>`__
+into three keys, including one for Flax Linen ``Dropout``.
+
+.. codediff::
+  :title_left: No Dropout
+  :title_right: With Dropout
+  :sync:
+
+  root_key = jax.random.PRNGKey(seed=0)
+  main_key, params_key = jax.random.split(key=root_key)
+  ---
+  root_key = jax.random.PRNGKey(seed=0)
+  main_key, params_key, dropout_key = jax.random.split(key=root_key, num=3) #!
+
+**Note:** In Flax, you provide *PRNG streams* with *names*, so that you can use them later
+in your :meth:`flax.linen.Module`. For example, you pass the stream ``'params'``
+for initializing parameters, and ``'dropout'`` for applying
+:meth:`flax.linen.Dropout`.
+
+Define your model with ``Dropout``
+**********************************
+
+To create a model with dropout:
+
+* Subclass :meth:`flax.linen.Module`, and then use
+  :meth:`flax.linen.Dropout` to add a dropout layer. Recall that
+  :meth:`flax.linen.Module` is the
+  `base class for all neural network modules <https://flax.readthedocs.io/en/latest/api_reference/flax.linen.html#module>`__,
+  and all layers and models are subclassed from it.
+
+* In :meth:`flax.linen.Dropout`, the ``deterministic`` argument is required to
+  be passed as a keyword argument, either:
+
+  * When constructing the :meth:`flax.linen.Module`; or
+  * When calling :meth:`flax.linen.init()` or :meth:`flax.linen.apply()` on a constructed ``Module``. (Refer to :meth:`flax.linen.module.merge_param` for more details.)
+
+* Because ``deterministic`` is a boolean:
+
+  * If it's set to ``False``, the inputs are masked (that is, set to zero) with
+    a probability set by ``rate``. And the remaining inputs are scaled by
+    ``1 / (1 - rate)``, which ensures that the means of the inputs are
+    preserved.
+  * If it's set to ``True``, no mask is applied (the dropout is turned off),
+    and the inputs are returned as-is.
+
+A common pattern is to accept a ``training`` (or ``train``) argument (a boolean)
+in the parent Flax ``Module``, and use it to enable or disable dropout (as
+demonstrated in later sections of this guide). In other machine learning
+frameworks, like PyTorch or TensorFlow (Keras), this is specified via a
+mutable state or a call flag (for example, in
+`torch.nn.Module.eval <https://pytorch.org/docs/stable/generated/torch.nn.Module.html#torch.nn.Module.eval>`__
+or ``tf.keras.Model`` by setting the
+`training <https://www.tensorflow.org/api_docs/python/tf/keras/Model#call>`__ flag).
+
+**Note:** Flax provides an implicit way of handling PRNG key streams via Flax
+:meth:`flax.linen.Module`'s :meth:`flax.linen.Module.make_rng` method.
+This allows you to split off a fresh PRNG key inside Flax Modules (or their
+sub-Modules) from the PRNG stream. The ``make_rng`` method guarantees to provide a
+unique key each time you call it. Internally, :meth:`flax.linen.Dropout` makes
+use of :meth:`flax.linen.Module.make_rng` to create a key for dropout. You can
+check out the
+`source code <https://github.com/google/flax/blob/5714e57a0dc8146eb58a7a06ed768ed3a17672f9/flax/linen/stochastic.py#L72>`__.
+In short, :meth:`flax.linen.Module.make_rng` *guarantees full reproducibility*.
+
+.. codediff::
+  :title_left: No Dropout
+  :title_right: With Dropout
+  :sync:
+
+  class MyModel(nn.Module):
+    num_neurons: int
+
+    @nn.compact
+    def __call__(self, x):
+      x = nn.Dense(self.num_neurons)(x)
+
+
+
+      return x
+  ---
+  class MyModel(nn.Module):
+    num_neurons: int
+
+    @nn.compact
+    def __call__(self, x, training: bool): #!
+      x = nn.Dense(self.num_neurons)(x)
+      # Set the dropout layer with a `rate` of 50%. #!
+      # When the `deterministic` flag is `True`, dropout is turned off. #!
+      x = nn.Dropout(rate=0.5, deterministic=not training)(x) #!
+      return x
+
+Initialize the model
+********************
+
+After creating your model:
+
+* Instantiate the model.
+* Then, in the :meth:`flax.linen.init()` call, set ``training=False``.
+* Finally, extract the ``params`` from the
+  `variable dictionary <https://flax.readthedocs.io/en/latest/api_reference/flax.linen.html#module-flax.core.variables>`__.
+
+Here, the main difference between the code without Flax ``Dropout``
+and with ``Dropout`` is that the ``training`` (or ``train``) argument must be
+provided if you need dropout enabled.
+
+.. codediff::
+  :title_left: No Dropout
+  :title_right: With Dropout
+  :sync:
+
+  my_model = MyModel(num_neurons=3)
+  x = jnp.empty((3, 4, 4))
+
+  variables = my_model.init(params_key, x)
+  params = variables['params']
+  ---
+  my_model = MyModel(num_neurons=3)
+  x = jnp.empty((3, 4, 4))
+  # Dropout is disabled with `training=False` (that is, `deterministic=True`). #!
+  variables = my_model.init(params_key, x, training=False) #!
+  params = variables['params']
+
+Perform the forward pass during training
+****************************************
+
+When using :meth:`flax.linen.apply()` to run your model:
+
+* Pass ``training=True`` to :meth:`flax.linen.apply()`.
+* Then, to draw PRNG keys during the forward pass (with dropout), provide a PRNG key
+  to seed the ``'dropout'`` stream when you call :meth:`flax.linen.apply()`.
+
+.. codediff::
+  :title_left: No Dropout
+  :title_right: With Dropout
+  :sync:
+
+  # No need to pass the `training` and `rngs` flags.
+  y = my_model.apply({'params': params}, x)
+  ---
+  # Dropout is enabled with `training=True` (that is, `deterministic=False`). #!
+  y = my_model.apply({'params': params}, x, training=True, rngs={'dropout': dropout_key}) #!
+
+Here, the main difference between the code without Flax ``Dropout``
+and with ``Dropout`` is that the ``training`` (or ``train``) and ``rngs``
+arguments must be provided if you need dropout enabled.
+
+During evaluation, use the above code with no dropout enabled (this means you do
+not have to pass a RNG either).
+
+Training
+********
+
+This section explains how to amend your code inside the training step function if
+you have dropout enabled.
+
+**Note:** Recall that Flax has a common pattern where you create a dataclass
+that represents the whole training state, including parameters and the optimizer
+state. Then, you can pass a single parameter, ``state: TrainState``, to
+the training step function. Refer to the
+:meth:`flax.training.train_state.TrainState` API docs to learn more.
+
+* First, add a ``key`` field to a custom :meth:`flax.training.train_state.TrainState` class.
+* Then, pass the ``key`` value—in this case, the ``dropout_key``—to the :meth:`train_state.TrainState.create` method.
+
+.. codediff::
+  :title_left: No Dropout
+  :title_right: With Dropout
+  :sync:
+
+  from flax.training import train_state
+
+
+
+
+  state = train_state.TrainState.create(
+    apply_fn=my_model.apply,
+    params=params,
+
+    tx=optax.adam(1e-3)
+  )
+  ---
+  from flax.training import train_state
+
+  class TrainState(train_state.TrainState): #!
+    key: jax.random.KeyArray #!
+
+  state = TrainState.create( #!
+    apply_fn=my_model.apply,
+    params=params,
+    key=dropout_key, #!
+    tx=optax.adam(1e-3)
+  )
+
+* Next, in the Flax training step function, ``train_step``, generate a new PRNG
+  key from the ``dropout_key`` to apply dropout at each step. This can be done with one of the following:
+
+  * `jax.random.split() <https://jax.readthedocs.io/en/latest/_autosummary/jax.random.split.html>`__; or
+  * `jax.random.fold_in() <https://jax.readthedocs.io/en/latest/_autosummary/jax.random.fold_in.html>`__
+
+  Using ``jax.random.fold_in()`` is generally faster. When you use
+  ``jax.random.split()`` you split off a PRNG key that can be reused
+  afterwards. However, using ``jax.random.fold_in()`` makes sure to 1) fold in
+  unique data; and 2) can result in longer sequences of PRNG streams.
+
+* Finally, when performing the forward pass, pass the new PRNG key to ``state.apply_fn()``
+  as an extra parameter.
+
+.. codediff::
+  :title_left: No Dropout
+  :title_right: With Dropout
+  :sync:
+
+  @jax.jit
+  def train_step(state: TrainState, batch):
+
+    def loss_fn(params):
+      logits = state.apply_fn(
+        {'params': params},
+        x=batch['image'],
+
+
+        )
+      loss = optax.softmax_cross_entropy_with_integer_labels(
+        logits=logits, labels=batch['label'])
+      return loss, logits
+    grad_fn = jax.value_and_grad(loss_fn, has_aux=True)
+    (loss, logits), grads = grad_fn(state.params)
+    state = state.apply_gradients(grads=grads)
+    return state
+
+  ---
+  @jax.jit
+  def train_step(state: TrainState, batch, dropout_key): #!
+    dropout_train_key = jax.random.fold_in(key=dropout_key, data=state.step) #!
+    def loss_fn(params):
+      logits = state.apply_fn(
+        {'params': params},
+        x=batch['image'],
+        training=True, #!
+        rngs={'dropout': dropout_train_key} #!
+        )
+      loss = optax.softmax_cross_entropy_with_integer_labels(
+        logits=logits, labels=batch['label'])
+      return loss, logits
+    grad_fn = jax.value_and_grad(loss_fn, has_aux=True)
+    (loss, logits), grads = grad_fn(state.params)
+    state = state.apply_gradients(grads=grads)
+    return state
+
+Flax examples with dropout
+**************************
+
+* A `Transformer-based model <https://github.com/google/flax/blob/main/examples/wmt/models.py>`__
+  trained on the WMT Machine Translation dataset. This example uses dropout and attention dropout.
+
+More Flax examples that use randomness
+**************************************
+
+* Applying word dropout to a batch of input IDs in a
+  `text classification <https://github.com/google/flax/blob/main/examples/sst2/models.py>`__
+  context.
+
+* Defining a prediction token in a decoder of a
+  `sequence-to-sequence model <https://github.com/google/flax/blob/main/examples/seq2seq/models.py>`__.

docs/guides/index.rst

@@ -8,6 +8,7 @@ Guides
    flax_basics
    state_params
    setup_or_nncompact
+   dropout
    batch_norm
    model_surgery
    transfer_learning