Add log_multimodal param to GRPOConfig and RLOOConfig to control image logging

[apardyl]

What does this PR do?

This PR introduces a log_multimodal option to GRPOConfig and RLOOConfig, which controls whether multimodal data is logged alongside completions.

When multiple high-resolution images are used during training, the log can quickly grow in size. While viewing images together with completions is very useful for debugging, disabling this feature for longer training runs can significantly reduce (cloud) storage usage.

The log_multimodal option defaults to True, preserving the current behavior.

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Note

Low Risk
Low risk: gated logging-only behavior change with default preserving existing output, affecting only whether image data is gathered/stored in completion logs.

Overview
Adds a new log_multimodal option to GRPOConfig and RLOOConfig (default True) to control whether images/multimodal payloads are included in completion logging.

Updates GRPOTrainer and RLOOTrainer to honor this flag by skipping the images gather/append step when disabled, reducing log size for high-resolution multimodal training runs.

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