Simple training of `model` for 1 epoch of `dl` using optim `opt` and loss function `loss_func`.

def train_epoch(model:nn.Module, dl:DataLoader, opt:optim.Optimizer, loss_func:LossFunction)->None:
    "Simple training of `model` for 1 epoch of `dl` using optim `opt` and loss function `loss_func`."
    model.train()
    for xb,yb in dl:
        loss = loss_func(model(xb), yb)
        loss.backward()
        opt.step()
        opt.zero_grad()