fix unflatten_dense_tensor when there is empty tensor inside (#50321)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/50321
Quantization team reported that when there are two empty tensors are replicated among ranks, the two empty tensors start to share storage after resizing.
The root cause is unflatten_dense_tensor unflattened the empty tensor as view of flat tensor and thus share storage with other tensors.
This PR is trying to avoid unflatten the empty tensor as view of flat tensor so that empty tensor will not share storage with other tensors.
Test Plan: unit test
Reviewed By: pritamdamania87
Differential Revision: D25859503
fbshipit-source-id: 5b760b31af6ed2b66bb22954cba8d1514f389cca
