[data] fix: update parquet_files type check to support multi-file input

[looput]

What does this PR do?

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API and Usage Example

When using xargs to perform SFT training on multiple datasets, the command is:

torchrun --nnodes=1 --nproc_per_node=8 -m verl.trainer.fsdp_sft_trainer \
    data.train_files=["data_path_1","data_path_2"] \
    ...

The existing code checks whether the input is a list, but the actual parsed result is a ListConfig, causing an extra level of nesting and leading to an error.

        if not isinstance(parquet_files, list):
            parquet_files = [parquet_files]

Therefore, the condition is updated to isinstance(parquet_files, list | ListConfig).

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[gemini-code-assist]

Code Review

This pull request addresses a type-checking issue in MultiTurnSFTDataset by updating the condition to correctly handle omegaconf.ListConfig inputs for parquet_files. This prevents incorrect list nesting and ensures that multiple file paths provided via configuration are processed correctly. The change is a good improvement. I've added one high-severity comment regarding an unhandled edge case that could lead to a crash.