[Feature Request] use MultiSystems in post_fp_vasp function

[erichuangyf]

Summary
Add MultiSystems support while posting vasp jobs

Detailed Description
I am currently investigating the feasibility of incorporating multiple vacancy structures with different formulas into a single sys_configs tag. However, I encountered error message #306 due to inconsistent atom numbers in this mixed system setup. This behavior was expected, but I have identified a potential solution to address it.

To resolve the issue, I suggest modifying the post_fp_vasp function in dpgen/dpgen/generator/run.py:3914 as follows: replace all_sys = None with all_sys = dpdata.MultiSystems(type_map=jdata["type_map"]). This adjustment utilizes the MultiSystems functionality from the dpdata library, effectively preventing the single-labeled system complaining about formula difference.

While this change successfully resolves the problem, I have concerns regarding potential risks associated with introducing multiple systems in one fp task. Therefore, I would like to inquire about any potential harm or drawbacks in this approach. If there are no significant concerns, I kindly request your consideration in applying this code change, enabling the mixing of vacancy structures into one training system.

[njzjz]

This seems reasonable to me. But we need to support MultiSystems for dpgen collect before this change.

[njzjz]

But we need to support MultiSystems for dpgen collect before this change.

Done in #1422.

MultiSystems can be introduced then.