Metadata for input/output formats in model file proto. (#15252)

Metadata for input/output formats in model file proto. (#15252)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15252

We would like to extend the model file format to include strongly type, semantic information
about the model inputs and outputs.

The goal is for a user to be able to consider a model file like a function with
a well defined API describing what the inputs and outputs would be.

Reviewed By: dzhulgakov

Differential Revision: D13009915

fbshipit-source-id: 5df124a876ad03c05fbdaacae0eab659637734c1