CheckpointWrapper state_dict fix (#77224)
- Uses state dict / load state dict hooks to ensure that modules wrapped with `CheckpointWrapper` can be loaded into non-checkpointed wrapped module.
This is because a training run can use activation checkpointing, then we can recover `state_dict`, and a future run may not want to wrap modules with activation checkpointing or decide to change activation checkpoint wrapping structure. To support this, we add hooks to remove / add the relevant prefix as needed.
Tests are added to ensure we can load into CheckpointWrapper module as well as local module from CheckpointWrapper-wrapped module. state_dict with FSDP is also verified.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77224
Approved by: https://github.com/zhaojuanmao
