Split FutureNCCL's CUDA-specific parts from generic future logic (#48504)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48504
This commit is part of a stack that reworks FutureNCCL in order to extract a generic CUDA-aware Future subclass. The stack deliberately breaks up this transition into elementary changes, to make it easier to verify that the behavior is preserved (or to highlight how it gets changed).
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FutureNCCL isn't just adding CUDA support to ivalue::Future, it's also reimplementing a lot of the latter's logic (by overriding plenty of its methods). That's brittle, as whenever a new method is added to ivalue::Future there's a risk of forgetting to add it to FutureNCCL, and in such a case calling this method on FutureNCCL would defer to the base class and give inconsistent results (e.g., future not being completed when it actually is). This _is already happening_, for example with the waitAndThrow or hasError, which are not implemented by FutureNCCL. In addition, this creates duplication between the two classes, which could lead to inconsistencies of behavior, bugs, missing features, ...
The best solution would be to keep the core future logic in ivalue::Future, and have _only_ the CUDA additions in FutureNCCL. That's what we're going to do, in two steps. In this commit, I'll split the CUDA features into separate hooks, which are called by FutureNCCL's other methods. In the next commit, I'll remove these latter methods, and invoke the hooks directly from ivalue::Future.
ghstack-source-id: 118180025
Test Plan: Unit tests
Reviewed By: mrshenli
Differential Revision: D25180534
fbshipit-source-id: 7b3cd374aee78f6c07104daec793c4d248404c61
