text: Title: how to find the end token in _sample_mask?
  Body: https://github.com/zihangdai/xlnet/blob/5cd50bc451436e188a8e7fea15358d5a8c916b72/data_utils.py#L369
  
  why isn't _is_start_piece(sp.IdToPiece(seg[end].item()))? Should it check the end of n-grams?
  