Remove NCCL dependency from PythonFutureWrapper (#48495)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48495
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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PythonFutureWrapper needs to provide a GIL-aware way to extract tensors from an IValue of type PyObject. Since this was only used by FutureNCCL it was guarded by #ifdef USE_C10D_NCCL. However, we will need to use it with CUDA-aware futures other than the NCCL one. This might have been achieved simply by replacing USE_C10D_NCCL with USE_CUDA, but I wanted to clean this up better.
We're dealing with two independent dimensions: C++-vs-Python and CPU-vs-CUDA. To make the code more modular, the two dimensions should be dealt with by orthogonal solutions: the user setting a custom callback to handle Python, and the subclass being CUDA-aware. Mixing these two axes makes it more complicated.
Another reason for changing how this works is that later on, when we'll introduce multi-device support, we'll need to extract dataptrs for other reasons too (rather than just recording streams with the caching allocator), namely to inspect the value to determine which devices it resides on.
ghstack-source-id: 118180038
Test Plan: Unit tests
Reviewed By: mrshenli
Differential Revision: D25177560
fbshipit-source-id: 3a424610c1ea191e8371ffee0a26d62639895884
