move `flatten_dense_tensors` and `unflatten_dense_tensors` to Native (#58006)
Summary:
https://github.com/pytorch/pytorch/issues/55240
CC ngimel
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58006
Reviewed By: agolynski
Differential Revision: D28386749
Pulled By: ngimel
fbshipit-source-id: 4860c35d5ff95bcc38a243d7001180e7bd536314
