Fix potential bugs in accuracy.py (open-mmlab#1496)

Author: Johnson-Wang

mmseg/models/losses/accuracy.py

@@ -45,14 +45,18 @@ def accuracy(pred, target, topk=1, thresh=None, ignore_index=None):
     if thresh is not None:
         # Only prediction values larger than thresh are counted as correct
         correct = correct & (pred_value > thresh).t()
-    correct = correct[:, target != ignore_index]
+    if ignore_index is not None:
+        correct = correct[:, target != ignore_index]
     res = []
     eps = torch.finfo(torch.float32).eps
     for k in topk:
         # Avoid causing ZeroDivisionError when all pixels
         # of an image are ignored
         correct_k = correct[:k].reshape(-1).float().sum(0, keepdim=True) + eps
-        total_num = target[target != ignore_index].numel() + eps
+        if ignore_index is not None:
+            total_num = target[target != ignore_index].numel() + eps
+        else:
+            total_num = target.numel() + eps
         res.append(correct_k.mul_(100.0 / total_num))
     return res[0] if return_single else res