message: Switch op_hint.py to use _set_attr.
  This is in preparation for enabling the C API. Modifying an op's NodeDef
  directly has no effect with the C API enabled.
diff: @@ -119,8 +119,10 @@ class OpHint(object):
  def _setattr(self, dest_op, name, value):
  tensor_value = _ops.convert_to_tensor(value)
  - dest_op.op.node_def.attr[name].tensor.CopyFrom(
  - tensor_value.op.node_def.attr["value"].tensor)
  + # pylint: disable=protected-access
  + dest_op.op._set_attr(name, _attr_value_pb2.AttrValue(
  + tensor=tensor_value.op.node_def.attr["value"].tensor))
  + # pylint: enable=protected-access
  def add_inputs(self, *args):
  """Add a sequence of inputs to the function invocation.
  