add an indexer to SymbolicShape (#45450)
Summary:
A convenience indexer into `SymbolicShape`s
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45450
Reviewed By: ZolotukhinM
Differential Revision: D23971758
Pulled By: Krovatkin
fbshipit-source-id: 1f18c5f89f579072f6bf467809ea9471bf42bc2d
