[model, cfg] fix:Allow list of modules to be passed to Lora target_modules

[thvasilo]

What does this PR do?

Fixes #5222

PeftConfig from PEFT supports passing a list of strings as target_modules.

However, target_modules is annotated as Optional[str] in verl/workers/config/model.py which leads to runtime errors when trying to override its default "all-linear" value with e.g. a list encoded as JSON string e.g. ["q_proj","v_proj"].

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Test

Added unit test that caught the error (allows replication if you switch the type back to Optional[str])

API and Usage Example

Demonstrate how the API changes if any, and provide usage example(s) if possible.

# Users should now be able to pass
python -m verl.trainer.main_ppo \
    actor_rollout_ref.model.target_modules='["k_proj","o_proj"]'

Design & Code Changes

I tried setting the type to Optional[str | list[str]] but was getting an OmegaConf error because it doesn't support unions in structured configs: omry/omegaconf#144

Instead I opted for Any and added type checks in __post_init__.

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[gemini-code-assist]

Code Review

This pull request correctly addresses an issue where target_modules could not be a list of strings. The approach of changing the type to Any and adding validation in __post_init__ is a good solution for the OmegaConf limitation. My review includes two main points for improvement: first, to make the validation logic more robust by raising TypeError instead of using assert, as assertions can be disabled. Second, to add unit tests for this new validation logic, as it is currently not covered by the new tests. These changes will improve the correctness and reliability of the configuration handling.