[CP] Register KV cache allgather buffer with symmetric memory (#24040)

Signed-off-by: wangfakang <fakangwang@gmail.com>

python/sglang/srt/layers/utils/cp_utils.py

@@ -179,12 +179,15 @@ def cp_all_gather_reorganized_into_tensor_kv_cache(
         input_tensor = F.pad(input_tensor, padding, mode="constant", value=0)
 
     # Create output tensor with proper shape for all dimensions
-    input_tensor_full = torch.empty(
-        max_len * cp_size,
-        *input_tensor.shape[1:],
-        device=input_tensor.device,
-        dtype=input_tensor.dtype,
-    )
+    with use_symmetric_memory(
+        get_attention_cp_group(), disabled=not is_allocation_symmetric()
+    ):
+        input_tensor_full = torch.empty(
+            max_len * cp_size,
+            *input_tensor.shape[1:],
+            device=input_tensor.device,
+            dtype=input_tensor.dtype,
+        )
 
     get_attention_cp_group().cp_all_gather_into_tensor_async(
         input_tensor_full, input_tensor, stream