Multiple outputs is pretty straightforward. Just return mutiple values in the forward() method of your net.
def forward(self, x):
# Do your stuff here
...
x1 = F.log_softmax(x) # class probabilities
x2 = ... # bounding box calculation
return x1, x2
Using these two outputs, you can define two different loss functions and just add them.
out1, out2 = model(data)
loss1 = criterion1(out1, target1)
loss2 = criterion2(out2, target2)
loss = loss1 + loss2
loss.backward()