[BUG] finetune with fparam/aparam

[njzjz]

Discussed in #3255

^{Originally posted by chtchelkatchev February 10, 2024}
I pretrained the deepmd potential with fparam using the descriptor se_atten_v2 and deepmd=2.2.7. Then an attempt was made to finetune this potential on a new database with fparam, which led to an error message: "... in _prepare_feed_dict assert fparam is not None ... AssertionError". Without fparam fintuning works well. The ussue appears only with fparam. Does finetuning work with fparam?

[njzjz]

We have to handle fparam/aparam in dp test, dp model-devi, and finetune. Thus, it will be more reasonable to add a new method to do it.