[MM] Move Qwen3Omni MRoPE impl to model file (#26608)

Signed-off-by: Roger Wang <hey@rogerw.io>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

vllm/model_executor/models/vision.py

@@ -499,3 +499,40 @@ def run_dp_sharded_mrope_vision_model(
         "Found unassigned embeddings"
     )
     return out_embeddings
+
+
+def get_llm_pos_ids_for_vision(
+    start_idx: int,
+    vision_idx: int,
+    spatial_merge_size: int,
+    t_index: list[int],
+    grid_hs: torch.Tensor,
+    grid_ws: torch.Tensor,
+) -> torch.Tensor:
+    llm_pos_ids_list = []
+    llm_grid_h = grid_hs[vision_idx] // spatial_merge_size
+    llm_grid_w = grid_ws[vision_idx] // spatial_merge_size
+    h_index = (
+        torch.arange(llm_grid_h)
+        .view(1, -1, 1)
+        .expand(len(t_index), -1, llm_grid_w)
+        .flatten()
+    )
+    w_index = (
+        torch.arange(llm_grid_w)
+        .view(1, 1, -1)
+        .expand(len(t_index), llm_grid_h, -1)
+        .flatten()
+    )
+    t_index_tensor = (
+        torch.Tensor(t_index)
+        .to(llm_grid_h.device)
+        .view(-1, 1)
+        .expand(-1, llm_grid_h * llm_grid_w)
+        .long()
+        .flatten()
+    )
+    _llm_pos_ids = torch.stack([t_index_tensor, h_index, w_index])
+    llm_pos_ids_list.append(_llm_pos_ids + start_idx)
+    llm_pos_ids = torch.cat(llm_pos_ids_list, dim=1)
+    return llm_pos_ids