Separate c10::Symbol header from list of interned strings (#69406)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69406
Most files that include `interned_strings.h` don't actually depend on
anything generated from `FORALL_NS_SYMBOLS` yet because they're in a
single file you need to recompile whenever a new symbol is added. Here
I move the class definition into a separate file so this doesn't
happen.
Test Plan: Imported from OSS
Reviewed By: zou3519
Differential Revision: D32923637
Pulled By: albanD
fbshipit-source-id: 6e488cbfcfe2c041a99d9ff22e167dbddf3f46d7
