Autograd tests for Transforms MFCC (#1415)

Co-authored-by: krishnakalyan3 <skalyan@cloudera.com>

test/torchaudio_unittest/transforms/autograd_test_impl.py

@@ -60,3 +60,10 @@ class AutogradTestMixin(TestBaseMixin):
         transform = T.MelSpectrogram(sample_rate=sample_rate)
         waveform = get_whitenoise(sample_rate=sample_rate, duration=0.05, n_channels=2)
         self.assert_grad(transform, [waveform], nondet_tol=1e-10)
+
+    @parameterized.expand([(False, ), (True, )])
+    def test_mfcc(self, log_mels):
+        sample_rate = 8000
+        transform = T.MFCC(sample_rate=sample_rate, log_mels=log_mels)
+        waveform = get_whitenoise(sample_rate=sample_rate, duration=0.05, n_channels=2)
+        self.assert_grad(transform, [waveform])