Fix HF dataset uploads to include validation splits if present

[bxyu-nvidia]

[fsiino-nvidia]

Validation splits have been uploaded for 3 datasets: https://huggingface.co/datasets/nvidia/Nemotron-RL-agent-workplace_assistant/tree/main https://huggingface.co/datasets/nvidia/Nemotron-RL-instruction_following-structured_outputs/tree/main https://huggingface.co/datasets/nvidia/Nemotron-RL-math-OpenMathReasoning/tree/main