Create a singular "to HF" checkpoint script

There are multiple checkpoint formats:
* dtensorv2 (safe tensors)
* dtensorv2 (DCP)
* mcore (DCP)

and we currently provide two checkpoint conversion scripts that users sometimes mistakenly apply (use mcore conversion script for Dtensor V2 DCP b/c the file looks the same).

Since there is enough metadata in the checkpoint directory, we should provide a single conversion entrypoint that automatically converts all checkpoint types.