[jit] Fix dict type serialization (#32569)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/32569
If the dict's contained types cannot be inferred from its contents (for
example, `Dict[str, Tensor]` vs. `Dict[str, Optional[Tensor]]`), we must
explicitly annotate the type.
Also this removes some special handling that omits annotations on empty
containers that have the default type. It makes the code more complex
for not too much value, and was wrong for dicts anyway.
Test Plan: Imported from OSS
Differential Revision: D19551016
Pulled By: suo
fbshipit-source-id: c529b112e72c10f509a6bc0f5876644caa1be967
