[ZeRO][BE] Clean up ZeRO tests (#73842)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/73842
**Overview**
This cleans up the `ZeroRedundancyOptimizer` tests. I apologize for strong formatting changes mixed in with actually-beneficial changes. It was convenient to unify the formatting while doing a deep comb through the full test file.
The main non-formatting changes include:
- Using `parametrize` instead of manually including `for` loops over possible argument values
- Removing the `DEVICE` global variable, which was used only for the `TestZeroRedundancyOptimizerSingleRank` tests, in favor of consistent usage of `self.device` in both `TestZeroRedundancyOptimizerSingleRank` and `TestZeroRedundancyOptimizerDistributed`
- Moving `assert ... == ...` to `self.assertEqual(..., ...)` when the assert is part of the test's correctness
- Removing the `if self.rank >= self.world_size or (torch.cuda.is_available() and torch.cuda.device_count() < 2):` conditional guards in favor of `common_distributed.skip_if_no_gpu` for `TestZeroRedundancyOptimizerDistributed`
- For `TestZeroRedundancyOptimizerDistributed`, `self.device` is `torch.device(self.rank)` if CUDA is available, while `self.world_size` is at least 2, even if `torch.cuda.device_count() == 1`.
- The problematic case is exactly when `torch.cuda.device_count() == 1` but `self.world_size == 2` since then calling `self.device` on rank 1 will error. The existing conditional guard prevented this case for some tests, but it was not used consistently (e.g. `test_multiple_groups()`), which is most likely the reason for the hangs and resulting test flakiness. (From my experience landing the recent ZeRO constructor changes, the Windows environment uses a world size of 2 but only has 1 device available.)
- A more robust solution is to always use the `skip_if_no_gpu` decorator as long as the test uses `self.device` and CUDA is available. This is in line with the recommended SPSD usage of ZeRO.
- Renaming `test_multiple_groups()` to `test_nondefault_process_group()`
- The existing `test_multiple_groups()` was slightly misnamed. Also, it is only nontrivial for a world size of (at least) 4 since it tests using a process group including only even ranks. It was marked as flaky on Windows, and I believe this is because of the world size and `torch.cuda.device_count()` mismatch. Now, the test only uses GPU if there are enough available and falls back to CPU otherwise, which is safe since the test uses Gloo backend.
- There was also a duplicated section, which I was unsure how to non-naively de-duplicate. The top half and bottom half are identical even though they claim to target fitting into the broadcast bucket and not fitting into the broadcast bucket:
https://github.com/pytorch/pytorch/blob/1d497114e7e13822df403082975d4b212d836207/test/distributed/optim/test_zero_redundancy_optimizer.py#L658-L684
- Changing `_test_zero_model_parallel()` to not use CPU
- This is my own fault, having introduced this inefficiency last summer. It makes more sense to simply designate one of the two GPUs for a process to be its default device rather than routing through CPU.
**Questions**
- How might we limit the runs for `test_ddp_zero_overlap()`? Because it parameterizes over many values, it contributes significantly to the time-to-signal. However, it is an experimental feature, so it is not critical that the tests run every time.
Test Plan: Imported from OSS
Reviewed By: rohan-varma
Differential Revision: D34675709
Pulled By: awgu
fbshipit-source-id: 71ce9ac968fb34415cd65206855b4bb5e67754fb
(cherry picked from commit 34e3dd0a184318ea9f63a1ee20cd14b111af3501)
