Beef up documentation on DispatchKey.h (#33011)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33011
I also reordered some of the keys in non-semantic ways to make the
organizational grouping mroe clear.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Differential Revision: D19796584
Pulled By: ezyang
fbshipit-source-id: 3083abadb47e9f382b9fbe981af0b34203c6ea4d
