Figures are never closed in CM metrics
Matplotlib complains that figures are left open when creating images of Confusion Matrices.

The exact warning is:

> avalanche/evaluation/metrics.py:292: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam `figure.max_open_warning`).

We should consider closing them in a "finally" block.

diff --git a/avalanche/evaluation/metrics.py b/avalanche/evaluation/metrics.py
index d7a5c6df..fd6c4711 100644
--- a/avalanche/evaluation/metrics.py
+++ b/avalanche/evaluation/metrics.py
@@ -309,7 +309,8 @@ class CM(object):
 
         buf = io.BytesIO()
         fig.savefig(buf, format='jpg', dpi=50)
-        # fig.savefig("temp.jpg", format='jpg')
+        plt.close(fig)
+
         buf.seek(0)
         image = PIL.Image.open(buf)
         image = ToTensor()(image)
['avalanche/evaluation/metrics.py']