add logging in sparse_to_dense_mask when skipping (#19945)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19945
this logging was removed in D6888977, but i think it could be useful to debug upstream data issue to check the violating index
Reviewed By: xianjiec
Differential Revision: D15127887
fbshipit-source-id: 4ad7eceefcd063bf45bc190a4c0d458a089c918a
