[FSDP] Add FlatParameter to track the information of a flat parameter (#69241)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69241
Implement FlatParameter to track the information of a flat parameter, including the sharding information.
Test Plan: CI
Reviewed By: zhaojuanmao
Differential Revision: D32432503
fbshipit-source-id: b4aabba6cef29e825b45869895709c79e69c211d
(cherry picked from commit 0e5505f70b69ecfeeec2c70688d4fc8f4a35e417)
