[Feature Request] Mim download config without checkpoint

[austinmw]

Describe the feature

Download config without weights, e.g.

mim download --config <config_name> --no-checkpoint

Motivation
Often may just want config file, especially if the config already includes the web address to download the checkpoint at runtime

[zhouzaida]

Hi @austinmw , sorry for my late reply. I have submitted a PR to implement this feature in #216.