Fix GPU memory leak introduced by correctness checking (#830)
Summary:
After calling `del`, we need to explicitly call `torch.cuda.empty_cache()` to free up the GPU memory used.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/830
Reviewed By: desertfire
Differential Revision: D35187195
Pulled By: xuzhao9
fbshipit-source-id: 06c5eaa6081f887ea2956a0d0692f41dff33958b
