Add metadata for torch jit TracedModules. (#17640)

Add metadata for torch jit TracedModules. (#17640)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17640

Pull Request resolved: https://github.com/pytorch/pytorch/pull/17311

I've extended our model metadata framework in this diff to support
traced modules as well. Re-used a lot of components from the previous
implementation of ScriptModule metadata.

Tracing is a little different from Scripting since you can't just create a
subclass of TopLevelTraceModule (type returned by torch.jit.trace) and attach
metadata the way we did for ScriptModule. As a result, I've introduced a
separate API torch.fb.jit_trace which returns an instance of
TracedModuleWithMetadata which is a subclass of TopLevelTracedModule. As a
result, we can now attach metadata to this instance.

Reviewed By: dzhulgakov

Differential Revision: D14117966

fbshipit-source-id: 3eee5eef733cb8d6a219c02e2f41d08698eca326