[JIT] Add `__prepare_scriptable__` duck typing to allow replacing nn.modules with scriptable preparations (#45645) (#49242)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49242
Fixes https://github.com/pytorch/pytorch/issues/45072
As discussed with zdevito gchanan cpuhrsch and suo, this change allows developers to create custom preparations for their modules before scripting. This is done by adding a `__prepare_scriptable__` method to a module which returns the prepared scriptable module out-of-place. It does not expand the API surface for end users.
Prior art by jamesr66a: https://github.com/pytorch/pytorch/pull/42244
Test Plan: Imported from OSS
Reviewed By: dongreenberg
Differential Revision: D25500303
fbshipit-source-id: d3ec9005de27d8882fc29d02f0d08acd2a5c6b2c
