Implement NetDef <--> JIT IR converters. (#16967)

Implement NetDef <--> JIT IR converters. (#16967)

Summary:
Currently the converters are very straightforward, i.e. there is no code for trying to
preserve semantics, we're purely perform conversion from one format to another.

Two things that we might want to add/change:
1. Add semantic conversion as well (but probably it would be a good idea to keep
it separate as a temporary thing).
2. Make sure we don't mess with value names, as they are crucial for current
uses of NetDefs.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16967

Differential Revision: D14062537

Pulled By: ZolotukhinM

fbshipit-source-id: 88b184ee7276779e5e9152b149d69857515ad98a