text: Title: Is it possible to use templates without {"mask"}?
  Body: I am using OpenPrompt on UnifiedQA (https://github.com/allenai/unifiedqa), which generates the answer using T5 without a mask token. 
  I tried a template without {"mask"}, but this is not allowed in OpenPrompt.
  I use the template {"placeholder":"text_a"} \n {"placeholder":"text_b"} {"mask"}, following the instruction in UnifiedQA, but the generation results are most empty.
  Is there a way to use T5 generation (instead of mask filling) in OpenPrompt? 
  
  like this:
  ![image](https://user-images.githubusercontent.com/24226286/179470374-e183b90b-c9c3-4229-ad8d-e80c50070ed5.png)
  