[bugfix] Fix position embedding processing for Qwen2.5-VL

In the `RLHFDataset.__getitem__` method, a bug was identified in how
multimodal position IDs (3D in Qwen2.5-VL) are determined. Previously,
the code checked for `self.image_key in row_dict` to decide whether to
use multimodal position IDs. However, since `self.image_key` is popped
from `row_dict` during image token expansion, this check incorrectly
fails for subsequent operations.

This causes the VL model to use incorrect position IDs, resulting in
significant performance degradation.

<img width="349" alt="image"
src="https://github.com/user-attachments/assets/79790bbf-239e-4667-a2c5-d63d91d63165"
/>


The fix introduces an explicit `is_multi_modal` flag to properly track
multimodal content throughout the processing pipeline.

Co-authored-by: songyifan <songyifan3@xiaomi.com>