text: Title: How can we support text_to_instance running in parallel?
  Body: Though we have a multi-process data-loader, we use for loops in [predictor](https://github.com/allenai/allennlp/blob/main/allennlp/predictors/predictor.py#L299).
  
  It will make the process hard and slow when we have many inputs( especially when this predictor as a server).
  
  I mean can we use or add some method to make this call(`text_to_instance`) support multi-process ?
  
  @epwalsh  @dirkgr maybe you could help ? 
  