Fix get_gpu_memory_info in non-cuda builds (#21054)
Summary:
#21024 broke master
cc akyrola
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21054
Reviewed By: akyrola
Differential Revision: D15533406
Pulled By: bddppq
fbshipit-source-id: 0dcfa0ce865e109b46280ef1786dbc7a8af30739
