[rollout] fix: qwen2_vl position_ids shape mismatch

[m-Just]

What does this PR do?

Fix qwen2_vl position_id shape mismatch: verl/models/transformers/qwen2_vl.py:process_position_ids expects position_ids to have a shape of (4, batch_size, seq_length) but verl/experimental/agent_loop/agent_loop.py:generate_sequences returns (batch_size, 3, seq_length) (which will be transposed to (3, batch_size, seq_length)), ignoring the text dimension. This PR follows the relevant code in verl/utils/dataset/rl_dataset.py to fix the issue.

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[gemini-code-assist]

Code Review

This pull request aims to fix a shape mismatch for position_ids in qwen2_vl. While it correctly identifies that an additional dimension for text position IDs is needed, the resulting tensor shape is still incorrect for the model's expectations, which would lead to a runtime error. I've provided a critical comment with a suggested fix that corrects the tensor dimensions. This fix also requires a small change in the _postprocess method, which I've detailed in the comment.