Save checkpoint to disk for API with new save layout

[eracah]

What does this PR do?

1. This PR introduces an API for saving the common objects one would save in a checkpoint:

   • save_model_to_disk
   • save_optim_to_disk
   • save_composer_metadata_to_disk
   • save_resumption_state_to_disk
     These functions make sure to save each object into its own directory

2. Introduces a dataclass, CheckpointSaveOptions for configuring checkpoint saving

3. Introduces a save to disk workhorse function, save_checkpoint_to_disk that handles saving all components of a checkpoint. Uses the API in 1. to save each component into its own separate file

Extras:

• adds an init_state helper function for tests to initialize a State object for tests

[mvpatel2000]

LGTM, would be helpful to have someone check though, many moving parts here