Revise VGGish pipeline test again (#3551)

Summary:
Pull Request resolved: https://github.com/pytorch/audio/pull/3551

Restores VGGish pipeline test to be a function rather than class.

Reviewed By: mthrok

Differential Revision: D48236197

fbshipit-source-id: 25ac19d87a7a0964a9c3f7552037cd6c21dc38a9

test/integration_tests/prototype/vggish_pipeline_test.py

-import unittest
-
 import torchaudio
 from torchaudio.prototype.pipelines import VGGISH
 
 
-class VGGishPipelineTest(unittest.TestCase):
-    def test_vggish(self):
+def test_vggish():
     input_sr = VGGISH.sample_rate
     input_proc = VGGISH.get_input_processor()
     model = VGGISH.get_model()