Fix demucs jit error with type annotation (#856)
Summary:
Demucs jit test failed with error "Unknown type name 'torch.Tensor'" (https://app.circleci.com/pipelines/github/pytorch/benchmark/4092/workflows/4d99d970-de28-41f0-bab9-0f0ece1f9aa9/jobs/4209).
This can be fixed by importing the torch library and adding a type annotation to the code.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/856
Reviewed By: samdow
Differential Revision: D35555763
Pulled By: xuzhao9
fbshipit-source-id: 3653bfcb20ee7fb818a100fceb882910ae549d18
