Enable nested default hooks (#70932)
Summary:
When default hooks are set, they are pushed onto a stack.
When nesting context-manager, only the inner-most hooks will
be applied.
There is special care needed to update the TLS code. See also https://github.com/pytorch/pytorch/issues/70940 (i.e. do we need to be storing the enabled flag as well?)
Fixes https://github.com/pytorch/pytorch/issues/70134
Pull Request resolved: https://github.com/pytorch/pytorch/pull/70932
Reviewed By: mruberry
Differential Revision: D33530370
Pulled By: albanD
fbshipit-source-id: 3197d585d77563f36c175d3949115a0776b309f4
