reseed all Generators in Dataloader's _worker_loop() -- via GC (#107131)
Alternative to https://github.com/pytorch/pytorch/pull/107034, implements @ezyang 's suggestion from https://github.com/pytorch/pytorch/pull/107034#discussion_r1292857201.
This PR addresses https://fb.workplace.com/groups/pytorch.oss.dev/posts/1699944830430051 and does a bunch of stacked changes:
- Make `Generator` class support GC;this makes all `Generator` instances tracked and accessile through Python's GC.
- Use the GC to retrieve all existing Generator instances in Dataloader's `_worker_loop` and re-seed them: this extends what is already applied to the global/default Generator, which is already re-seeded.
~TODO: a bit of docs and justification, which I'll do if this PR is mergeable.~ -- Done
CC @albanD @ezyang as previously discussed
BC-Breaking Note
-------------------
We now re-seed all `Generator` instances within the `Dataloader` workers' loop to ensure that their RNG is different across workers.
Previously, the RNG of user-defined `Generators` would be the same across workers, which could lead to wrong training procedures. This only affects user-defined `Generators`, not the default `Generator` (which was already re-seeded).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107131
Approved by: https://github.com/ezyang
