[rollout] fix: qwen2_vl position_ids shape mismatch (volcengine#3653)

### 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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Author: m-Just

verl/experimental/agent_loop/agent_loop.py

@@ -589,14 +589,20 @@ async def _run_agent_loop(
                 video_grid_thw = multi_modal_inputs.get("video_grid_thw")
                 second_per_grid_ts = multi_modal_inputs.get("second_per_grid_ts")
 
-                position_ids = get_rope_index(
+                vision_position_ids = get_rope_index(
                     self.processor,
                     input_ids=input_ids.squeeze(0),
                     image_grid_thw=image_grid_thw,
                     video_grid_thw=video_grid_thw,
                     second_per_grid_ts=second_per_grid_ts,
                     attention_mask=attention_mask.squeeze(0),
                 ).unsqueeze(0)  # (1, 3, seq_len)
+
+                valid_mask = attention_mask[0].bool()
+                text_position_ids = torch.ones((1, len(input_ids[0])), dtype=torch.long)
+                text_position_ids[0, valid_mask] = torch.arange(valid_mask.sum().item())
+                text_position_ids = text_position_ids.unsqueeze(0)
+                position_ids = torch.cat((text_position_ids, vision_position_ids), dim=1)  # (1, 4, seq_length)
             else:
                 position_ids = compute_position_id_with_mask(attention_mask)  # (1, seq_len)
             enable_async_reward = (