message: Remove failing assertion
  Mixing v1/v2 TensorShapes seems fine except for this one assertion. I've verified that the object-based checkpointing tests pass with enable_v2_behavior() now (while failing on this assertion before). It was never an issue when setting the environmental variable, since everything saw a consistent TensorShape class.
diff: @@ -44,7 +44,6 @@ def graph_placeholder(dtype, shape, name=None):
  dtype_value = attr_value_pb2.AttrValue(type=dtype.as_datatype_enum)
  if isinstance(shape, (list, tuple)):
  shape = tensor_shape.TensorShape(shape)
  - assert isinstance(shape, tensor_shape.TensorShape)
  shape = attr_value_pb2.AttrValue(shape=shape.as_proto())
  g = ops.get_default_graph()
  with ops.name_scope(name, "placeholder", []) as name:
  