message: Keras RNNs - correctly set the dropout mask
  Summary:
  Make sure the same mask is used across the sequence.
  Fixes T26006
  Test Plan: All tests pass, added hw tests.
  Reviewers: #tensorflow, hakons, jakeh, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved
  Maniphest Tasks: T26006
diff: @@ -211,9 +211,15 @@ class _PopnnRNN(Layer):
  if not training:
  return inputs
  + # Apply the same dropout mask across the sequence - this function is called
  + # when the inputs is shaped as [S, B, N].
  + noise_shape = inputs.get_shape().as_list()
  + noise_shape[0] = 1
  +
  return rand_ops.dropout(inputs,
  seed=self._dropout_seed,
  rate=self._dropout,
  + noise_shape=noise_shape,
  name=self.name + "_dropout")
  