Fix CpmTokenizer for training/finetuning CPM model
When CpmTokenizer extends XLNetTokenizer, it works for inference/generation, but model in training/finetuning will failed after some steps, its error is random like:

    RuntimeError: CUDA error: device-side assert triggered
    terminate called after throwing an instance of 'std::runtime_error'
    what():  NCCL error in: /pytorch/torch/lib/c10d/../c10d/NCCLUtils.hpp:136, unhandled cuda error, NCCL version 2.7.8,

failed when in different tensor operation every time, very strange.
after extend XLNetTokenizerFast, inference/training/finetuning all working.
