chore: import upstream snapshot with attribution

decoding/GAD/fairseq/modules/layer_drop.py

@@ -0,0 +1,44 @@
+# Copyright (c) Facebook, Inc. and its affiliates.
+#
+# This source code is licensed under the MIT license found in the
+# LICENSE file in the root directory of this source tree.
+"""
+LayerDrop as described in https://arxiv.org/abs/1909.11556.
+"""
+
+import torch
+import torch.nn as nn
+
+
+class LayerDropModuleList(nn.ModuleList):
+    """
+    A LayerDrop implementation based on :class:`torch.nn.ModuleList`.
+
+    We refresh the choice of which layers to drop every time we iterate
+    over the LayerDropModuleList instance. During evaluation we always
+    iterate over all layers.
+
+    Usage::
+
+        layers = LayerDropList(p=0.5, modules=[layer1, layer2, layer3])
+        for layer in layers:  # this might iterate over layers 1 and 3
+            x = layer(x)
+        for layer in layers:  # this might iterate over all layers
+            x = layer(x)
+        for layer in layers:  # this might not iterate over any layers
+            x = layer(x)
+
+    Args:
+        p (float): probability of dropping out each layer
+        modules (iterable, optional): an iterable of modules to add
+    """
+
+    def __init__(self, p, modules=None):
+        super().__init__(modules)
+        self.p = p
+
+    def __iter__(self):
+        dropout_probs = torch.empty(len(self)).uniform_()
+        for i, m in enumerate(super().__iter__()):
+            if not self.training or (dropout_probs[i] > self.p):
+                yield m