[RELAND] Eager autocasting, out-of-place ops only (with MSVC 2017 fix) (#35011)
Summary:
https://github.com/pytorch/pytorch/pull/32140 was approved and merged, but [reverted](https://github.com/pytorch/pytorch/commit/d0577e19f09a32a68f0f3faed635dec72971b019) because it broke builds with versions of Visual Studio older than 15.8 that were not represented in public CI. The build failures were caused by a [known VS bug](https://developercommunity.visualstudio.com/content/problem/27729/allow-function-with-internal-linkage-as-template-n.html), fixed in versions 15.8 and newer.
The present PR reverts the revert (restoring https://github.com/pytorch/pytorch/pull/32140 's diffs) and adds a workaround to enable compilation with VS < 15.8. The workaround isn't pretty, but it's guarded by macros such that it's only used when compiling with VS < 15.8. All other builds compile with the same code/control flow as was merged in https://github.com/pytorch/pytorch/pull/32140.
Original description of https://github.com/pytorch/pytorch/pull/32140:
> Initial integration of eager autocasting, supporting out-of-place ops only for easier review.
Relevant issue/RFC: https://github.com/pytorch/pytorch/issues/25081
> In-place ops and ops with user-supplied out=... can certainly be supported as well (my initial WIP https://github.com/pytorch/pytorch/issues/29552 handled many) but require substantially more complex special casing in the autocasting backend and tests. Support for these ops (much of which has already been written) will be broken into later PRs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35011
Differential Revision: D20541921
Pulled By: ezyang
fbshipit-source-id: abb5488dca8620b0daac4306ebf2bb47fc36e4f5
