Functionality/pickling for commhooks (#79334)
This PR addresses issue address #75666.
Stateful communication hook now can be saved and reloaded to resume training.
Current PR adds the functionality for PowerSGD communication hook and tests that communication hook can be properly saved and restored.
PowerSGD implementation uses ``__slots__``, as a result introduced __getstate__ and __setstate__ methods are implemented to work with `__slots__` and not` __dict__`.
`__getstate__ `
Returns:
A dictionary that represents a ``PowerSGDState`` which will be pickled and saved.
``process_group`` is non-serializable and excluded from a returned state.
`__setstate__`
Takes a provided ``state`` and retrieves ``PowerSGDState``.
``process_group`` is set to default with a proper warning issued to a user.
Unit test
A hook-independent `_test_hook_pickling` is added with this PR, as well as `test_ddp_hook_pickling_powerSGD`, which tests `powerSGD`’s ability to be saved and reloaded.
Currently, the test creates a ddp model with a provided hook, trains it for 10 epochs and saves model’s state and hook’s state.
During reloading, unit test makes sure that a warning was logged (only one warning and the proper one). It then proceeds to check that reloaded hook and original hook are the same. Finally, it checks that a hook’s state was properly initialized:
- it compares slot values (all, but 2: `process_group` and `rng`) for original and reloaded state
- it checks that process group was set to a default group
- it checks that a random state was restored properly with np.testing.assert_array_equal, because `rng` is an instance of `np.random.RandomState`, represented by a tuple. One of entries is of `ndarray dtype[uint32]` type and `np.testing.assert_array_equal` is used for assertion.
Future To-Do:
- Implement similar __getstate__ and __setstate__ for other stateful communication hooks
- Add appropriate tests
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79334
Approved by: https://github.com/rohan-varma, https://github.com/awgu
