Add list len refinement (#55926)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55926
This is necessary for code like conv2d where we wish to share a generic convolution shape function logic with that of conv2d but for conv2d always infer the output is dimension 4. I'm also hoping the refinement algorithm here could be refactored out and used to support refining tensor types from user annotations. i have a length comment explaining how this works, and the logic outside of data structures is pretty small and contained. Additionally, you might check out https://fb.quip.com/X7EVAdQ99Zzm for a very similar description of how to refine values based on comparison operators.
Test Plan: Imported from OSS
Reviewed By: ZolotukhinM
Differential Revision: D27750997
Pulled By: eellison
fbshipit-source-id: d962415af519ac37ebc9de88f2e1ea60a1374f7c
