Fix a bug in loop unrolling (#21239)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21239
ghimport-source-id: 68256b752be795b32ab3f426848ed1d64fc5ea3e
Reviewed By: suo
Differential Revision: D15590901
Pulled By: zdevito
fbshipit-source-id: 8700aab723d4486fd20d3414df8160b36a3cc5da
