fix lint warning (#2027)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2027

Error message
```
  Warning (FLAKE8) F401
    'torch.export.exported_program.OutputKind' imported but unused
    See https://www.flake8rules.com/rules/F401.html.

          42  |)
          43  |from torch._export.passes import ReplaceViewOpsWithViewCopyOpsPass
          44  |from torch.export import ExportedProgram
    >>>   45  |from torch.export.exported_program import (
          46  |    _get_updated_range_constraints,
          47  |    ConstantArgument,
          48  |    ExportGraphSignature,
```

Reviewed By: lucylq

Differential Revision: D54011437

fbshipit-source-id: ae9c38220324e2c91a8afc5c145bbc8441e817e1