Fix errors when use verl to train GLM4.1v model

[kaln27]

• Support glm4v load from AutoModelForVision2Seq
• Set glm4v model's _checkpoint_conversion_mapping attribute from None to empty dict {}

What does this PR do?

When use verl to train GLM4.1v model with GRPO, there are several small errors.
Here is how to fix them:

• support glm4v load using AutoModelForVision2Seq
• verl treat _checkpoint_conversion_mapping as a dict. But right now is None, which will abort the program. I also found that almost every model which don't need checkpoint convert have a empty dict.

Fixes # (issue)

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[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: auto, glm4v

[kaln27]

@ArthurZucker
Hi Arthur, would you mind to review this PR?
Thank your !