Fix reduction of cross_entropy

Summary:
Reduction of cross_entropy is always set to "mean".
Fixed to receive the reduction.

Pull Request resolved: #3454

Reviewed By: zhanghang1989

Differential Revision: D31052191

Pulled By: ppwwyyxx

fbshipit-source-id: 0bd992cac13c67a772fac8539ab5cc4f7c8e5c3c

Author: Sukjun Hwang

detectron2/layers/wrappers.py

@@ -30,7 +30,7 @@ def cross_entropy(input, target, *, reduction="mean", **kwargs):
     """
     if target.numel() == 0 and reduction == "mean":
         return input.sum() * 0.0  # connect the gradient
-    return F.cross_entropy(input, target, **kwargs)
+    return F.cross_entropy(input, target, reduction=reduction, **kwargs)
 
 
 class _NewEmptyTensorOp(torch.autograd.Function):