Pass reference to parent future in callbacks (#57635)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/57635
Note: this PR looks massive, but it's just one simple change, codemodded many times.
In many cases, a callback needs to access the value/error produced by the parent future. In Python this was easy because the callback was invoked with the parent future as argument, and could thus inspect it. In C++ the callbacks didn't take any arguments, thus in many cases we worked around this by capturing the future in its own callback. This is risky (leads to reference cycle and thus memory leak) and must be done carefully (spoiler: sometimes we weren't).
ghstack-source-id: 128296580
Test Plan: CI
Reviewed By: wanchaol
Differential Revision: D28178783
fbshipit-source-id: 6de02c4568be42123372edc008f630d5ddae0081
