message: Add RegisterGradient's init Error
  RegisterGradient can return Type Error, so added it to docs.
diff: @@ -2728,6 +2728,9 @@ class RegisterGradient(object):
  Args:
  op_type: The string type of an operation. This corresponds to the
  `OpDef.name` field for the proto that defines the operation.
  +
  + Raises:
  + TypeError: If `op_type` is not string.
  """
  if not isinstance(op_type, six.string_types):
  raise TypeError("op_type must be a string")
  