[package] add mock/extern hooks (#58000)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58000
Directly overriding save_extern and save_mock may mess with our
invariants in weird ways. This is less pronounced now, but once we
switch to graph-based dependency management things will get broken
subtly if people fail to call `super()`.
Better to add hook support to reflect that really you can only do a side
effect. Also has the bonus that people are likely familiar with it from
`nn.Module` hooks.
Differential Revision: D28339191
Test Plan: Imported from OSS
Reviewed By: SplitInfinity
Pulled By: suo
fbshipit-source-id: 63ffd39d2dcb1a7524f3c2c6a23bd399e754cc44
