[state_dict][6/N] Change API names to avoid conflict and simplify the API signatures (#111120)
`state_dict` is a very common variable name people use to represent a local
state_dict and `load_state_dict` conflicts with DCP's `load_state_dict`.
This PR changes `state_dict` to `get_state_dict`. `get_state_dict` is more close to what is this API does -- users use the API to get the current state_dict for saving or for loading (passed to DCP for loading in-place)..
This PR also changes `load_state_dict` to `set_state_dict`. `set_state_dict` is less ideal compared to `get_state_dict` but is symetric. We can still change the API name before it goes to beta.
This PR also simplies the API signatures. `model_only` is removed and `optim_only` only exists for `get_state_dict`.
Differential Revision: [D50213931](https://our.internmc.facebook.com/intern/diff/D50213931/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/111120
Approved by: https://github.com/wz337
ghstack dependencies: #111106, #111107, #111275, #111109, #111110
