block_diag dense (#33449)
Summary:
Add block_diag function for dense tensors, based on scipy.linalg.block_diag
Closes https://github.com/pytorch/pytorch/issues/31932
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33449
Differential Revision: D20943099
Pulled By: zou3519
fbshipit-source-id: 8b5c9476fb5af959aafa4169612c660396d9b717
