Added example for AmplitudeToDB transform (#2615)

Summary:
Added example for AmplitudeToDB transform as mentioned in issue https://github.com/pytorch/audio/issues/1564.

Pull Request resolved: https://github.com/pytorch/audio/pull/2615

Reviewed By: carolineechen

Differential Revision: D38743117

Pulled By: nateanl

fbshipit-source-id: bf0f760299f4777a4bca65da86359faa00b16207

torchaudio/transforms/_transforms.py

@@ -310,6 +310,11 @@ class AmplitudeToDB(torch.nn.Module):
             power being the elementwise square of the magnitude. (Default: ``'power'``)
         top_db (float or None, optional): minimum negative cut-off in decibels.  A reasonable
             number is 80. (Default: ``None``)
+
+    Example
+        >>> waveform, sample_rate = torchaudio.load("test.wav", normalize=True)
+        >>> transform = transforms.AmplitudeToDB(stype="amplitude", top_db=80)
+        >>> waveform_db = transform(waveform)
     """
     __constants__ = ["multiplier", "amin", "ref_value", "db_multiplier"]