Add mixup https://arxiv.org/abs/1710.09412 to `learn`.

def mixup(learn:Learner, alpha:float=0.4, stack_x:bool=False, stack_y:bool=True) -> Learner:
    "Add mixup https://arxiv.org/abs/1710.09412 to `learn`."
    learn.callback_fns.append(partial(MixUpCallback, alpha=alpha, stack_x=stack_x, stack_y=stack_y))
    return learn