title: if len(xi) > 0: TypeError: len() of unsized object MATPLOTLIB
body: <p>l have this matrix to plot :</p>
  
  <pre><code> 1.0887e+00 -2.9071e+00  2.2645e+00  ...   2.2798e-01  6.1273e-01  2.9771e+00
   2.4369e-01 -1.5935e+00  1.9538e+00  ...  -8.5572e-01  1.2877e+00  7.0469e-01
   2.4409e-01 -1.4260e+00  1.0422e+00  ...  -9.8817e-01  8.8775e-01  4.3492e-01
                  ...                   ⋱                   ...                
   1.5589e+00 -1.9514e+00  1.5131e+00  ...  -4.2420e-01  8.9567e-01  1.7066e+00
   9.5703e-01 -2.6811e+00  2.4477e+00  ...  -1.1068e-01  2.2908e+00  4.0621e+00
   8.5307e-01 -2.4534e+00  1.1746e+00  ...  -4.7006e-01  1.3858e+00  1.0349e+00
  [torch.FloatTensor of size 37x512]
  </code></pre>
  
  <p>When l apply the following :</p>
  
  <pre><code>import matplotlib.pylot as plt
  plt.hist(conv0,bins=100)
  </code></pre>
  
  <p>l get the following error  : </p>
  
  <pre><code>matplotlib/axes/_axes.py", line 6177, in hist
      if len(xi) &gt; 0:
  TypeError: len() of unsized object
  </code></pre>
  
  <p>What's wrong with my code ?</p>