[NNC] Make flatten transform in-place (#56629)
Summary:
Partial fix for https://github.com/pytorch/pytorch/issues/56157
This PR updates the `flatten` API in `LoopNest` to perform the flattening transformation in-place. After this transformation, the first loop in the input becomes the flattened loop.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/56629
Reviewed By: H-Huang
Differential Revision: D28004787
Pulled By: navahgar
fbshipit-source-id: 7474ae237fae3fff0cd1c64a276a8831dc5b7db0
