fix none

pytorch_lightning/trainer/supporters.py

@@ -11,6 +11,8 @@ class TensorRunningMean(object):
         >>> accum.last(), accum.mean()
         (None, None)
         >>> accum.append(torch.tensor(1.5))
+        >>> accum.last(), accum.mean()
+        (tensor(1.5000), tensor(1.5000))
         >>> accum.append(torch.tensor(2.5))
         >>> accum.last(), accum.mean()
         (tensor(2.5000), tensor(2.))
@@ -53,7 +55,7 @@ class TensorRunningMean(object):
             self.rotated = True
 
     def mean(self):
-        if not self.last_idx:
+        if self.last_idx is None:
             return None
         avg = self.memory.mean() if self.rotated else self.memory[:self.current_idx].mean()
         return avg