The three arguments are self, net(image), and label. self is implicit since you’re calling a method in Python.
You probably want the functional form of mse_loss instead of the module:
import torch.nn.functional as F
loss = F.mse_loss(net(image), label)
loss.backward()

You could use the module nn.MSELoss like:
loss_fn = nn.MSELoss()
loss = loss_fn(net(image), label)
loss.backward()