[JIT] remove prim::shape op (#34286)
Summary:
Desugar prim::shape to aten::size so that passes don't need to reason about both ops. Serialized models still resolve to `prim::shape` so this doesn't break BC.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34286
Differential Revision: D20316818
Pulled By: eellison
fbshipit-source-id: d1585687212843f51e9396e07c108f5c08017818
