message: Fix typo in docstring of is_tensor
  The isinstance builtin expects a tuple of types, not list.
diff: @@ -942,7 +942,7 @@ def is_tensor(x): # pylint: disable=invalid-name
  """Check whether `x` is of tensor type.
  Check whether an object is a tensor. This check is equivalent to calling
  - `isinstance(x, [tf.Tensor, tf.SparseTensor, tf.Variable])` and also checks
  + `isinstance(x, (tf.Tensor, tf.SparseTensor, tf.Variable))` and also checks
  if all the component variables of a MirroredVariable or a TowerLocalVariable
  are tensors.
  