Make Optimizer.state_dict() nondeterministic (#37347)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/36831.
Instead of using `id()`, an arbitrary yet consistent order-based index is used instead. This results in a deterministic output between runs.
I am not the biggest fan of using `nonlocal` (it appears to be used sparingly in the codebase) to get `start_index` between calls to `pack_group()`, but the alternatives had larger issues:
- Using the last value added to `param_mappings` would be ideal, but that only works if `dict` iteration order is consistent, and PyTorch currently supports Python <3.7.
- Using the maximum value added to `param_mappings` wouldn't have that issue but would not be constant time.
For testing, I confirmed that `test_optim.py` works before and after these changes. Randomizing the indices in `param_mappings` causes the tests to fail, which is further evidence these changes work. I'm not 100% if these tests are sufficient, but they're a start.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37347
Differential Revision: D21353820
Pulled By: vincentqb
fbshipit-source-id: e549f1f154833a461b1f4df6d07ad509aab34ea1
