[DCP][BE] Let FsspecWriter and FsspecReader inherit from FileSystemWriter and FileSystemReader (#118887)
There is no logic changed. However this PR dramatially reduces the effort to maintain filesystem-like storage backend. As we are going to enable fsspec, this is a must BE iteam.
Differential Revision: [D53318044](https://our.internmc.facebook.com/intern/diff/D53318044/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/118887
Approved by: https://github.com/wz337