[caffe2] Add __slots__ to all classes in schema.py (#47541)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47541
The profiler has guided us to `schema.py`. Since these `Field`s are used everywhere and in huge quantities, we can easily make some optimizations system wide by adding `__slots__`.
From StackOverflow, benefits include:
* faster attribute access.
* space savings in memory.
Read more: https://stackoverflow.com/a/28059785/
Reviewed By: dzhulgakov
Differential Revision: D24771078
fbshipit-source-id: 13f6064d367440069767131a433c820eabfe931b
