Auto create commandline args from yamls with fairseq-hydra-train

🚀 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?