fix reduce_util for 0-D tensor

Summary: For 0-D tensor, we should still be able to get 0 and -1 dimensions (treat it as a 1-D tensor with only one number)

Reviewed By: manuelcandales

Differential Revision: D48247370

fbshipit-source-id: 26a6a629055e8ac4a67075a479bd801e201982c7