Added stochastic depth layer

.github/CODEOWNERS

@@ -105,6 +105,8 @@
 /tensorflow_addons/layers/tests/esn_test.py @pedrolarben
 /tensorflow_addons/layers/snake.py @failure-to-thrive
 /tensorflow_addons/layers/tests/snake_test.py @failure-to-thrive
+/tensorflow_addons/layers/stochastic_depth.py @mhstadler @windqaq
+/tensorflow_addons/layers/tests/stochastic_depth_test.py @mhstadler @windqaq
 /tensorflow_addons/layers/noisy_dense.py @leonshams
 /tensorflow_addons/layers/tests/noisy_dense_test.py @leonshams
 

tensorflow_addons/layers/__init__.py

@@ -38,4 +38,5 @@
 from tensorflow_addons.layers.tlu import TLU
 from tensorflow_addons.layers.wrappers import WeightNormalization
 from tensorflow_addons.layers.esn import ESN
+from tensorflow_addons.layers.stochastic_depth import StochasticDepth
 from tensorflow_addons.layers.noisy_dense import NoisyDense

tensorflow_addons/layers/stochastic_depth.py

@@ -0,0 +1,88 @@
+import tensorflow as tf
+from typeguard import typechecked
+
+
+@tf.keras.utils.register_keras_serializable(package="Addons")
+class StochasticDepth(tf.keras.layers.Layer):
+    """Stochastic Depth layer.
+
+    Implements Stochastic Depth as described in
+    [Deep Networks with Stochastic Depth](https://arxiv.org/abs/1603.09382), to randomly drop residual branches
+    in residual architectures.
+
+    Usage:
+    Residual architectures with fixed depth, use residual branches that are merged back into the main network
+    by adding the residual branch back to the input:
+
+    >>> input = np.ones((1, 3, 3, 1), dtype = np.float32)
+    >>> residual = tf.keras.layers.Conv2D(1, 1)(input)
+    >>> output = tf.keras.layers.Add()([input, residual])
+    >>> output.shape
+    TensorShape([1, 3, 3, 1])
+
+    StochasticDepth acts as a drop-in replacement for the addition:
+
+    >>> input = np.ones((1, 3, 3, 1), dtype = np.float32)
+    >>> residual = tf.keras.layers.Conv2D(1, 1)(input)
+    >>> output = tfa.layers.StochasticDepth()([input, residual])
+    >>> output.shape
+    TensorShape([1, 3, 3, 1])
+
+    At train time, StochasticDepth returns:
+
+    $$
+    x[0] + b_l * x[1],
+    $$
+
+    where $b_l$ is a random Bernoulli variable with probability $P(b_l = 1) = p_l$
+
+    At test time, StochasticDepth rescales the activations of the residual branch based on the survival probability ($p_l$):
+
+    $$
+    x[0] + p_l * x[1]
+    $$
+
+    Arguments:
+        survival_probability: float, the probability of the residual branch being kept.
+
+    Call Arguments:
+        inputs:  List of `[shortcut, residual]` where `shortcut`, and `residual` are tensors of equal shape.
+
+    Output shape:
+        Equal to the shape of inputs `shortcut`, and `residual`
+    """
+
+    @typechecked
+    def __init__(self, survival_probability: float = 0.5, **kwargs):
+        super().__init__(**kwargs)
+
+        self.survival_probability = survival_probability
+
+    def call(self, x, training=None):
+        if not isinstance(x, list) or len(x) != 2:
+            raise ValueError("input must be a list of length 2.")
+
+        shortcut, residual = x
+
+        # Random bernoulli variable indicating whether the branch should be kept or not or not
+        b_l = tf.keras.backend.random_bernoulli([], p=self.survival_probability)
+
+        def _call_train():
+            return shortcut + b_l * residual
+
+        def _call_test():
+            return shortcut + self.survival_probability * residual
+
+        return tf.keras.backend.in_train_phase(
+            _call_train, _call_test, training=training
+        )
+
+    def compute_output_shape(self, input_shape):
+        return input_shape[0]
+
+    def get_config(self):
+        base_config = super().get_config()
+
+        config = {"survival_probability": self.survival_probability}
+
+        return {**base_config, **config}

tensorflow_addons/layers/tests/stochastic_depth_test.py

@@ -0,0 +1,58 @@
+import pytest
+import numpy as np
+import tensorflow as tf
+
+from tensorflow_addons.layers.stochastic_depth import StochasticDepth
+from tensorflow_addons.utils import test_utils
+
+_KEEP_SEED = 1111
+_DROP_SEED = 2222
+
+
+@pytest.mark.parametrize("seed", [_KEEP_SEED, _DROP_SEED])
+@pytest.mark.parametrize("training", [True, False])
+def stochastic_depth_test(seed, training):
+    np.random.seed(seed)
+    tf.random.set_seed(seed)
+
+    survival_probability = 0.5
+
+    shortcut = np.asarray([[0.2, 0.1, 0.4]]).astype(np.float32)
+    residual = np.asarray([[0.2, 0.4, 0.5]]).astype(np.float32)
+
+    if training:
+        if seed == _KEEP_SEED:
+            # shortcut + residual
+            expected_output = np.asarray([[0.4, 0.5, 0.9]]).astype(np.float32)
+        elif seed == _DROP_SEED:
+            # shortcut
+            expected_output = np.asarray([[0.2, 0.1, 0.4]]).astype(np.float32)
+    else:
+        # shortcut + p_l * residual
+        expected_output = np.asarray([[0.3, 0.3, 0.65]]).astype(np.float32)
+
+    test_utils.layer_test(
+        StochasticDepth,
+        kwargs={"survival_probability": survival_probability},
+        input_data=[shortcut, residual],
+        expected_output=expected_output,
+    )
+
+
+@pytest.mark.usefixtures("run_with_mixed_precision_policy")
+def test_with_mixed_precision_policy():
+    policy = tf.keras.mixed_precision.experimental.global_policy()
+
+    shortcut = np.asarray([[0.2, 0.1, 0.4]])
+    residual = np.asarray([[0.2, 0.4, 0.5]])
+
+    output = StochasticDepth()([shortcut, residual])
+
+    assert output.dtype == policy.compute_dtype
+
+
+def test_serialization():
+    stoch_depth = StochasticDepth(survival_probability=0.5)
+    serialized_stoch_depth = tf.keras.layers.serialize(stoch_depth)
+    new_layer = tf.keras.layers.deserialize(serialized_stoch_depth)
+    assert stoch_depth.get_config() == new_layer.get_config()