Fix support for model_init_kwargs in MiniLLM when passed as CLI JSON string

[albertvillanova]

Fix support for model_init_kwargs in MiniLLM when passed as CLI JSON string:

• Add missing model_init_kwargs to MiniLLMConfig _VALID_DICT_FIELDS

Note that MiniLLMConfig subclasses GRPOConfig, but it was extending the _VALID_DICT_FIELDS class attribute from TrainingArguments instead of the one from their direct parent. As a result, model_init_kwargs (which is defined in GRPOConfig) was not included in the valid dictionary fields for these configs, causing it to be ignored when passed as a CLI JSON string.

This PR ensures that model_init_kwargs is properly recognized and parsed in the MiniLLMConfig class. The main change is to ensure that this class references their base class GRPOConfig for valid dictionary fields instead of the external TrainingArguments class, improving consistency and maintainability.

Follow-up to:

• Fix support for model_init_kwargs in GKD/GOLD when passed as CLI JSON string #5266
• Support JSON string parsing of teacher_model_init_kwargs in MiniLLMConfig #5259

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Note

Low Risk
Low risk: a one-line config metadata change that only affects which CLI/JSON-specified fields are accepted and parsed for MiniLLMConfig.

Overview
Fixes MiniLLMConfig dictionary/CLI JSON parsing by building _VALID_DICT_FIELDS from its direct parent GRPOConfig instead of TrainingArguments, ensuring inherited fields like model_init_kwargs are no longer ignored when passed as JSON strings.

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[qgallouedec]

indeed