Improvements for symbolic AD (#14758)

Improvements for symbolic AD (#14758)

Summary:
**Review only the last commit.**

This commit adds a few optimizations to AD, that let us dramatically
reduce the number of sizes we capture from forward.

We now:
- collapse chains of SumToSize
- avoid capturing sizes of tensors that are captured anyway
- more aggressively DCE the reverse code
- run CSE on the primal code to deduplicate `aten::size` calls

cc zou3519 zdevito
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14758

Differential Revision: D13324440

Pulled By: zou3519

fbshipit-source-id: 45ccbc13605adcef2b461840c6089d3200000c72