pred = torch.tensor([1.0, 2.0, 5.0])
tgt  = torch.tensor([1.0, 3.0, 3.0])

nn.MSELoss(reduction='mean')(pred, tgt)   # tensor(1.6667)
nn.MSELoss(reduction='sum')(pred, tgt)    # tensor(5.)
nn.MSELoss(reduction='none')(pred, tgt)   # tensor([0., 1., 4.])