text: Title: detail about prefix tuning
  Body: In prefix_tuning_template.py file
  https://github.com/thunlp/OpenPrompt/blob/675545ce1f946aa186efda8e8640dbc29fd1159f/openprompt/prompts/prefix_tuning_template.py#L207
  
  The above code pad the attention_mask for extra prompt tokens.
  
  Why the function 'torch.zeros' is used here? Should we use 'torch.ones' here? 