Migrate state dict API to DSD

[mori360]

Migrate state dict API to DSD:

• load_from_full_model_state_dict -> set_model_state_dict
• get_full_model_state_dict -> get_model_state_dict
• load_from_full_optimizer_state_dict -> set_optimizer_state_dict
• get_full_optimizer_state_dict -> get_optimizer_state_dict

What is the purpose of this PR? Is it to

• add a new feature
• fix a bug
• update tests and/or documentation
• other (please add here)

Please link to any issues this PR addresses.

Changelog

What are the changes made in this PR?
*

Test plan

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• run pre-commit hooks and linters (make sure you've first installed via pre-commit install)
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UX

If your function changed a public API, please add a dummy example of what the user experience will look like when calling it.
Here is a docstring example
and a tutorial example

• I did not change any public API
• I have added an example to docs or docstrings

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/torchtune/1930

• 📄 Preview Python docs built from this PR

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❌ 3 New Failures

As of commit d4713f8 with merge base a1bcb97 ():

NEW FAILURES - The following jobs have failed:

• GPU tests / gpu_test (3.10, stable) (gh)
  tests/torchtune/training/test_distributed.py::TestFullyShardState::test_qlora_state_dict
• GPU tests / gpu_test (3.11, stable) (gh)
  tests/torchtune/training/test_distributed.py::TestFullyShardState::test_qlora_state_dict
• GPU tests / gpu_test (3.9, stable) (gh)
  tests/torchtune/training/test_distributed.py::TestFullyShardState::test_qlora_state_dict

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