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Describe the bug
merge_yi这个方法的实现有问题，如果一句话中出现了多个 <动词，一，动词>组合的话，会有溢出错误。

To Reproduce
Steps to reproduce the behavior:
case:
seg = [('奶嘴', 'n'), ('儿', 'n'), ('是不是', 'l'), ('平常', 'a'), ('咱', 'r'), ('煮', 'v'), ('一', 'm'), ('煮', 'v'), ('烫', 'v'), ('一', 'm'), ('烫', 'v'), ('啥', 'r'), ('的', 'uj'), ('沙', 'n'), ('杀', 'v'), ('小菌', 'n'), ('菌用', 'n'), ('的', 'uj'), ('干净', 'a'), (',', 'x')]

fixed version：

  def _merge_yi(seg):
      new_seg = []
      skip_next = False
      # function 1
      for i, (word, pos) in enumerate(seg):
          if skip_next:
              skip_next = False
              continue
          if i - 1 >= 0 and word == "一" and i + 1 < len(seg) and seg[i - 1][0] == seg[i + 1][0] and seg[i - 1][1] == "v":
              new_seg[-1] = (new_seg[-1][0] + "一" + seg[i + 1][0], new_seg[-1][1])
              skip_next = True
          else:
              new_seg.append((word, pos))
      seg = new_seg
      new_seg = []
      # function 2
      for i, (word, pos) in enumerate(seg):
          if new_seg and new_seg[-1][0] == "一":
              new_seg[-1] = (new_seg[-1][0] + word, new_seg[-1][1])
          else:
              new_seg.append((word, pos))
      return new_seg

  res = _merge_yi(seg)
  print(res)

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
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Environment (please complete the following information):

• OS: [e.g. Ubuntu]
• GCC/G++ Version [e.g. 8.3]
• Python Version [e.g. 3.7]
• PaddlePaddle Version [e.g. 2.0.0]
• Model Version [e.g. 2.0.0]
• GPU/DRIVER Informationo [e.g. Tesla V100-SXM2-32GB/440.64.00]
• CUDA/CUDNN Version [e.g. cuda-10.2]
• MKL Version
• TensorRT Version

Additional context
Add any other context about the problem here.