Add example for Spectrogram (#1566)

torchaudio/transforms.py

@@ -69,6 +69,12 @@ class Spectrogram(torch.nn.Module):
             This argument is only effective when ``power=None``. It is ignored for
             cases where ``power`` is a number as in those cases, the returned tensor is
             power spectrogram, which is a real-valued tensor.
+
+    Example
+        >>> waveform, sample_rate = torchaudio.load('test.wav', normalize=True)
+        >>> transform = torchaudio.transforms.Spectrogram(n_fft=800)
+        >>> spectrogram = transform(waveform)
+
     """
     __constants__ = ['n_fft', 'win_length', 'hop_length', 'pad', 'power', 'normalized']