class Acc:
    def __init__(self, ignore_index=1):
        self.ignore_index = ignore_index
    
    def __call__(self, pred, tgt):
        # both pred and tgt have shape (bs,seq_len)
        mask = tgt != self.ignore_index
        pred *= mask
        tgt *= mask
        correct = torch.eq(pred, tgt).all(1).sum()
        return correct.item()