[PyTorch Use MaybeOwned when promoting indices/offsets in embedding_bag (#120755)
We're currently doing two unnecessary reference count
operations in the case where promotion doesn't need to happen.
Differential Revision: [D54285999](https://our.internmc.facebook.com/intern/diff/D54285999/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/120755
Approved by: https://github.com/cyyever, https://github.com/Skylion007
ghstack dependencies: #120752
