message: Don't register the proto function twice. Needed for the TPUEstimator move to
  the Estimator repo.
diff: @@ -115,6 +115,8 @@ _RESERVED_PARAMS_KEYS = [_BATCH_SIZE_KEY, _CTX_KEY]
  # explicitly).
  _WRAP_INPUT_FN_INTO_WHILE_LOOP = False
  +if ops.get_to_proto_function(
  + '{}_{}'.format(_TPU_ESTIMATOR, _ITERATIONS_PER_LOOP_VAR)) is None:
  ops.register_proto_function(
  '{}_{}'.format(_TPU_ESTIMATOR, _ITERATIONS_PER_LOOP_VAR),
  proto_type=variable_pb2.VariableDef,
  