message: Fix typo in documentation for TPUEmbedding
  Current documentation says optimization_parameters must not be `None`. The exception raised in the constructor says otherwise.
diff: @@ -319,7 +319,7 @@ class TPUEmbedding(object):
  mode: `TRAINING` or `INFERENCE`.
  optimization_parameters: `AdagradParameters`, `AdamParameters`,
  `Stochasticgradientdescentparameters`. Must be set in training and must
  - not be `None` in inference.
  + be `None` in inference.
  tpu_embedding_test: A `bool`. Only used for testing.
  Raises:
  