fix lint warning (#2027)

Summary:

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

Author: cccclai

exir/program/_program.py

@@ -48,7 +48,6 @@
     ExportGraphSignature,
     InputKind,
     InputSpec,
-    OutputKind,
     OutputSpec,
     TensorArgument,
 )