Split CUDAFuture into a .h and a .cpp file (#56514)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56514
rohan-varma mentioned that having CUDAFuture entirely defined in a header meant having to rebuild a whole lot of things whenever it changed. In fact there's no reason not to use a .cpp file, so here I do so.
ghstack-source-id: 127035765
Test Plan: Unit tests
Reviewed By: rohan-varma, mrshenli
Differential Revision: D27861071
fbshipit-source-id: c209d54af9b52d3ad781db1b61f6fca02c637f32
