Add stop token visualizaiotn for the notebook

utils/visual.py

@@ -32,4 +32,4 @@ def plot_spectrogram(linear_output, audio):
     data = np.fromstring(fig.canvas.tostring_rgb(), dtype=np.uint8, sep='')
     data = data.reshape(fig.canvas.get_width_height()[::-1] + (3,))
     plt.close()
-    return data
+    return data