Calculate loss and metrics for a batch, call out to callbacks as necessary.

def loss_batch(model:nn.Module, xb:Tensor, yb:Tensor, loss_func:OptLossFunc=None, opt:OptOptimizer=None,
               cb_handler:Optional[CallbackHandler]=None)->Tuple[Union[Tensor,int,float,str]]:
    "Calculate loss and metrics for a batch, call out to callbacks as necessary."
    cb_handler = ifnone(cb_handler, CallbackHandler())
    if not is_listy(xb): xb = [xb]
    if not is_listy(yb): yb = [yb]
    out = model(*xb)
    out = cb_handler.on_loss_begin(out)

    if not loss_func: return to_detach(out), yb[0].detach()
    loss = loss_func(out, *yb)

    if opt is not None:
        loss,skip_bwd = cb_handler.on_backward_begin(loss)
        if not skip_bwd:                     loss.backward()
        if not cb_handler.on_backward_end(): opt.step()
        if not cb_handler.on_step_end():     opt.zero_grad()

    return loss.detach().cpu()