问题描述

我在使用 transformers.Trainer 训练我的模型时，我自定义了 compute_loss 函数和compute_metrics函数，我的模型是一个简单的二分类模型。

在自定义 compute_loss 时这样写的：

def compute_loss(self, model, inputs, return_outputs=False):
        """
        重写 Trainer.compute_loss:
        1) 提取字典中的 images, bboxes, locs, labels 等
        2) 用 vision_encoder 先处理图像，得到特征
        3) 用下游 model 做预测
        4) 计算并返回 loss
        """
        # 前向传播
        outputs, labels = model(**inputs)  # (bz, num_classes), or (bz*num_frames, num_classes)

        batch_size = inputs['labels'].shape[0]

        outputs = outputs.squeeze()  # (bz*num_frames)

        if batch_size == 1:
            outputs = outputs.unsqueeze(0)

        # 计算 loss
        loss = self.loss_func(outputs, labels.float())

        if self.state.global_step % 10 == 0 and self.state.global_step > 0:
            # 以50个step为间隔打印
            pred_probs = torch.sigmoid(outputs)
            preds = (pred_probs > 0.5).int()
            logger.info(f"[global_step={self.state.global_step}] preds={preds.tolist()} / labels={labels.tolist()} / loss={loss.item():.4f}")
            # compute metric
            accuracy = accuracy_score(labels.cpu().numpy(), preds.cpu().numpy())
            precision = precision_score(labels.cpu().numpy(), preds.cpu().numpy())
            recall = recall_score(labels.cpu().numpy(), preds.cpu().numpy())
            logger.info(f"[global_step={self.state.global_step}] accuracy={accuracy:.4f} / precision={precision:.4f} / recall={recall:.4f}")

        # 返回 (loss, outputs) 或者只返回 loss
        return (loss, outputs) if return_outputs else loss

于是就出现了报错，像这样的：

File "/opt/conda/lib/python3.9/site-packages/transformers/trainer.py", line 3754, in predict
    output = eval_loop(
  File "/opt/conda/lib/python3.9/site-packages/transformers/trainer.py", line 3966, in evaluation_loop
    metrics = self.compute_metrics(EvalPrediction(predictions=all_preds, label_ids=all_labels))
  File "/workspace/train/object_query/train.py", line 281, in compute_metrics
    correct_num = preds == labels
ValueError: operands could not be broadcast together with shapes (11720,) (12104,)
    output = eval_loop(
  File "/opt/conda/lib/python3.9/site-packages/transformers/trainer.py", line 3966, in evaluation_loop
    metrics = self.compute_metrics(EvalPrediction(predictions=all_preds, label_ids=all_labels))
  File "/workspace/train/object_query/train.py", line 281, in compute_metrics
    correct_num = preds == labels
ValueError: operands could not be broadcast together with shapes (11720,) (12104,)

原因

该问题是 transformers.Trainer 内部有一段对outputs的操作造成的：

if isinstance(outputs, dict):
    logits = tuple(v for k, v in outputs.items() if k not in ignore_keys + ["loss"])
else:
    logits = outputs[1:]

这里当 outputs 不是字典时，会把第一个位置的元素offset掉。

解决

Refer to here
所以，我们应该在返回那里这样写：

return (loss, {"label": outputs}) if return_outputs else loss