Remove unused subgraph vmap api (#52512)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52512
This API is not used at all, and is tricky to maintain. When we were using it last we ran into lifetime issues when using `Value *` as the key. In hind sight, we should have been using `value->unique()`, but regardless, this not being used and should be removed.
Test Plan: Imported from OSS
Reviewed By: navahgar
Differential Revision: D26696695
Pulled By: eellison
fbshipit-source-id: 97ed92e88ecab0085fabbac46573611666bf2420
