Add support for recursive compilation on Modules (#20708)
Summary:
Following on #19747, this implements most of the `torch.jit.script()` changes laid out in #20939.
Still to do:
* Accessing a method from Python does not add it as a `ScriptMethod` (so only `export`ed methods and `forward` are compiled)
* Calling a method other than `forward` on a submodule doesn't work
](https://our.intern.facebook.com/intern/diff/15546045/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20708
Pulled By: driazati
Differential Revision: D15546045
fbshipit-source-id: c2c8fe179088ffbdad47198e799a456560655b86
