update error_string when target is out of bound

[HydrogenSulfate]

PR types

Function optimization

PR changes

APIs

Describe

1. 更新cross_entropy边界检查时的报错代码，让输出的错误信息更清晰

• label超出上界的case

  import paddle
  logits = paddle.uniform(shape=[1, 10])
  label = paddle.to_tensor([100], dtype='int64')
  loss = paddle.nn.functional.cross_entropy(input=logits, label=label)
  
  # ---输出---
  Traceback (most recent call last):
    File "xxx/ce_loss_optimize.py", line 31, in <module>
      loss = paddle.nn.functional.cross_entropy(input=logits, label=label)
    File "xxx/python3.7/site-packages/paddle/nn/functional/loss.py", line 1662, in cross_entropy
      format(label_max.item()))
  ValueError: Target 100 is out of upper bound.

• label超出下界的case

  import paddle
  
  logits = paddle.uniform(shape=[1, 10])
  label = paddle.to_tensor([-2], dtype='int64')
  loss = paddle.nn.functional.cross_entropy(input=logits, label=label)
  
  # ---输出---
  Traceback (most recent call last):
    File "xxx/ce_loss_optimize.py", line 35, in <module>
      loss = paddle.nn.functional.cross_entropy(input=logits, label=label)
    File "xxx/python3.7/site-packages/paddle/nn/functional/loss.py", line 1659, in cross_entropy
      format(label_min.item()))
  ValueError: Target -2 is out of lower bound.

[paddle-bot-old]

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[chajchaj]

LGTM