message: Improved sequence_mask error message.
  When a tensor's shape is invalid, print that shape in the error message.
diff: @@ -2259,7 +2259,8 @@ def sequence_mask(lengths, maxlen=None, dtype=dtypes.bool, name=None):
  with ops.name_scope(name, "SequenceMask", [lengths, maxlen]):
  lengths = ops.convert_to_tensor(lengths)
  if lengths.get_shape().ndims != 1:
  - raise ValueError("lengths must be 1D for sequence_mask")
  + raise ValueError("lengths must be 1D for sequence_mask. Got shape %s" %
  + lengths.get_shape())
  if maxlen is None:
  maxlen = gen_math_ops._max(lengths, [0])
  