Fix cuda test failure (#3363)

Summary:
Fix https://github.com/pytorch/audio/issues/3361

When adding FunctionalCUDAOnlyTest, the class should inherit from `TestBaseMixin` instead of `Functional`

Pull Request resolved: https://github.com/pytorch/audio/pull/3363

Reviewed By: atalman, osalpekar

Differential Revision: D46112084

Pulled By: nateanl

fbshipit-source-id: 67c6472fda98cb718e0fc53ab248beda745feab5

test/torchaudio_unittest/functional/functional_impl.py

@@ -1234,7 +1234,7 @@ class FunctionalCPUOnly(TestBaseMixin):
         assert len(w) == 1
 
 
-class FunctionalCUDAOnly(Functional):
+class FunctionalCUDAOnly(TestBaseMixin):
     @nested_params(
         [torch.half, torch.float, torch.double],
         [torch.int32, torch.int64],