text: Title: Auto create commandline args from yamls  with fairseq-hydra-train
  Body: ## 🚀 Feature Request
  Though Fairseq is such a powerful framework, I got some problems when I used fairseq-hydra-train.
  If I want to define an arg in the config yaml file, I have to define it in some dataclass first.
  This is cumbersome and slow compared with the hydra's ability of auto-creating commandline args.
  Furthermore, this makes it impossible to use hydra's ability of multi-level nested parameters.
  How can I access the full ability of hydra with fairseq?
  