Added example for Loudness transform (#2641)

Summary:
Added example for Loudness transform (implemented in PR https://github.com/pytorch/audio/issues/2472) as mentioned in issue https://github.com/pytorch/audio/issues/1564.

Pull Request resolved: https://github.com/pytorch/audio/pull/2641

Reviewed By: nateanl

Differential Revision: D38907782

Pulled By: carolineechen

fbshipit-source-id: fd2bcc4bac3095a626ea9cf36cb70cb2bf003d63

torchaudio/transforms/_transforms.py

@@ -1293,6 +1293,11 @@ class Loudness(torch.nn.Module):
     Args:
         sample_rate (int): Sample rate of audio signal.
 
+    Example
+        >>> waveform, sample_rate = torchaudio.load("test.wav", normalize=True)
+        >>> transform = transforms.Loudness(sample_rate)
+        >>> loudness = transform(waveform)
+
     Reference:
         - https://www.itu.int/rec/R-REC-BS.1770-4-201510-I/en
     """