I recently move some code from pytorch 0.4.1 to pytorch 1.0.0 and found something wrong on torch.nn.MSELoss().
For example, I have two tensor named a and b, and I process them under  pytorch 0.4.1 and pytorch 1.0.0. Here is the code. Notice that the difference when I use reduction="elementwise_mean".
# code under pytorch 0.4.1
>>> a.shape
torch.Size([48, 3, 128, 128])
>>> b.shape
torch.Size([48, 3, 128, 128])
>>> loss1 = nn.MSELoss(reduction="sum")
>>> loss1(a,b)
tensor(1416916.3750, device='cuda:0', grad_fn=<SumBackward0>)
>>> loss2 = nn.MSELoss(reduction="elementwise_mean")
>>> loss2(a,b)
tensor(1416916.3750, device='cuda:0', grad_fn=<SumBackward0>)

# code under pytorch 1.0.0
>>> a.shape
torch.Size([48, 3, 128, 128])
>>> b.shape
torch.Size([48, 3, 128, 128])
>>> loss1 = nn.MSELoss(reduction="sum")
>>> loss1(a,b)
tensor(1416916.3750, device='cuda:0', grad_fn=<SumBackward0>)
>>> loss2 = nn.MSELoss(reduction="mean")
>>> loss2(a,b)
tensor(0.6006, device='cuda:0', grad_fn=<MeanBackward1>)

Is it really a bug in pytorch 0.4.1?