Rename legacy_dispatcher to native. (#45974)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45974
The term "legacy dispatcher" caused a bunch of confusion between
me and Sebastian when discussing what the intended semantics of
legacy dispatcher argument is. Legacy dispatcher argument implies
that you ought NOT to use it when you have use_c10_dispatcher: full;
but that's not really what's going on; legacy dispatcher API describes
the API that you write native:: functions (NativeFunctions.h) to.
Renaming it here makes this more clear.
I applied these seds:
```
git grep -l 'legacy_dispatcher' | xargs sed -i 's/legacy_dispatcher/native/g'
git grep -l 'legacydispatcher' | xargs sed -i 's/legacydispatcher/native/g'
git grep -l 'LegacyDispatcher' | xargs sed -i 's/LegacyDispatcher/Native/g'
```
and also grepped for "legacy" in tools/codegen and fixed documentation.
Signed-off-by: Edward Z. Yang <ezyang@fb.com>
Test Plan: Imported from OSS
Reviewed By: smessmer
Differential Revision: D24223101
Pulled By: ezyang
fbshipit-source-id: d1913b8b823b3b95e4546881bc0e876acfa881eb
