message: Fix misalignment of documentation in BahdanauAttention
  This fix fixes the misalignment of documentation in BahdanauAttention,
  as was specified in 28054. The issue seems to be that ` (optional)`
  should be placed after the `:` so that memory_sequence_length
  could be identified as an arg.
  This fix fixes 28054.
diff: @@ -937,7 +937,7 @@ class BahdanauAttention(_BaseAttentionMechanism):
  num_units: The depth of the query mechanism.
  memory: The memory to query; usually the output of an RNN encoder. This
  tensor should be shaped `[batch_size, max_time, ...]`.
  - memory_sequence_length (optional): Sequence lengths for the batch entries
  + memory_sequence_length: (optional) Sequence lengths for the batch entries
  in memory. If provided, the memory tensor rows are masked with zeros
  for values past the respective sequence lengths.
  normalize: Python boolean. Whether to normalize the energy term.
  