[primTorch] Make `prims.collapse` a real prim (#91748)
`prims.collapse` is currently just a plain python function wrapping
`prims.reshape`. This turns it into a real prim, and also factors out some of
the code duplicated with `_collapse_view_aten`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/91748
Approved by: https://github.com/lezcano, https://github.com/ngimel
