pytorch

88616349 - [state_dict][1/N] Implement the basic functions of distributed.checkpoint._state_dict (#105902)

[state_dict][1/N] Implement the basic functions of distributed.checkpoint._state_dict (#105902) This PR implements the basic functions of distributed.checkpoint._state_dict. This PR currently contains the flattening of optimizer state_dict which makes the PR too large. A later version may split it into 2 for a better code review. Differential Revision: [D47647719](https://our.internmc.facebook.com/intern/diff/D47647719/) **NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D47647719/)! Pull Request resolved: https://github.com/pytorch/pytorch/pull/105902 Approved by: https://github.com/wz337