Created by: zhiqiu

In PR #21359, we split VarBase from Variable. Currently, Variable (for static graph) and VarBase (for dygraph) are logically same type for developers. However, some of the internal APIs has small differences. This PR handles those APIs.

1. Some functions of Variable is not supported, such as detach(). Historically, these APIs are decorated by dygraph_only when VarBase have not been split from Variable. Basically, we should remove them. While, actually, now we keep these functions under class Variable for generating API docs only, since VarBase is not exposed and not mentioned in our official website. So, I add a fake_interface_only decorator for those APIs.

2. Some functions of Variable is not supported for dygraph Variable when VarBase have not been split from Variable, like lod_level. Historically, these APIs are decorated by dygraph_not_support. Since we have split VarBase, users cannot access these APIs by VarBase. So, I remove the dygraph_not_support decorators for those APIs.