don't check 0 elements for cat symbolic diff (#65751)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/65751
Fixes symbolic script grad formula for cat to correctly handle empty tensors
Test Plan: Existing tests
Reviewed By: eellison
Differential Revision: D31208364
fbshipit-source-id: d676d9abcc033b56076fa946f58f3db50034502d
