This is known behavior and is explained in the section Accumulating losses in the Migration Guide 7.
You would want this kind of behavior for example if you have different loss functions and would like to accumulate them:
total_loss = loss1 + loss2 + loss3
total_loss.backward()

Also you could accumulate the losses of several mini batches and backward them together:
for ...
    loss = criterion(...)
    total_loss += loss

total_loss.backward()

Therefore I’m not sure it would be a good idea to add a warning, since it’s used for the purpose of storing the computation graphs.