GRPO fix dataloader_num_workers value error in GRPOTrainer

[rolandtannous]

Problem

After resolving the initial state2 in unsloth PR 2943 attribute errors in GRPO training, users encountered a secondary validation error when initializing GRPOConfig:

ValueError: --dataloader_prefetch_factor can only be set when data is loaded in a different process, i.e. when --dataloader_num_workers > 1

This error occurs because Unsloth's RL patching system sets default values for dataloader optimization, including dataloader_prefetch_factor = 2 and dataloader_num_workers = 0. However, the Transformers library validation logic

However, the Transformers library transformers validation logic

if self.dataloader_num_workers == 0 and self.dataloader_prefetch_factor is not None:
            raise ValueError(
                "--dataloader_prefetch_factor can only be set when data is loaded in a different process, i.e."
                " when --dataloader_num_workers > 1."
            )

requires dataloader_num_workers > 1 when dataloader_prefetch_factor is specified, as prefetching only makes sense when data loading happens in separate worker processes.

The conflict arises from Unsloth's optimization defaults being incompatible with Transformers' validation requirements, preventing GRPO training from starting.

Solution

Modified the dataloader configuration logic in rl.py and set dataloader_num_workers default to 0, to ensure compatibility with Transformers validation.

Tests

We tested end-to-end the following GRPO notebooks to ensure both training and inference work correctly.
After applying the fixes in both unsloth PR 2943 and this PR, all notebooks now complete successfully without errors

Notebook	Training	Inference
Advanced Llama-3.1-(3B)-GRPO-Lora	✅	✅
Advanced_Llama3_2_(3B)_GRPO_LoRA	✅	✅
Phi-14B-GRPO	✅	✅
MMistral_v0.3_(7B)-GRPO	✅	✅
qwen3_4b-GRPO	✅	✅