Add subclass in model test classes (#727)

* add unittest in test_models

* update test method

* remove unittest main function
Co-authored-by: Ji Chen <jimchen90@devfair0160.h2.fair>

test/test_models.py

 import torch
 from torchaudio.models import Wav2Letter, _MelResNet
 
+from . import common_utils
 
-class TestWav2Letter:
+
+class TestWav2Letter(common_utils.TorchaudioTestCase):
 
     def test_waveform(self):
         batch_size = 2
@@ -31,7 +33,7 @@ class TestWav2Letter:
         assert out.size() == (batch_size, num_classes, 2)
 
 
-class TestMelResNet:
+class TestMelResNet(common_utils.TorchaudioTestCase):
 
     def test_waveform(self):