[Perf] Eliminate redundant SparseMatrix creation in gpt_oss_triton_kernels (#37683)

Signed-off-by: Xin Yang <xyangx@amazon.com>

vllm/model_executor/layers/fused_moe/gpt_oss_triton_kernels_moe.py

@@ -142,6 +142,33 @@ def legacy_routing_from_bitmatrix(
     return routing_data, gather_idx, scatter_idx
 
 
+def legacy_routing_from_sparsematrix(
+    sparse_logits: "SparseMatrix",
+    n_expts_tot: int,
+    n_expts_act: int,
+) -> tuple["RoutingData", "GatherIndx", "ScatterIndx"]:
+    """
+    Creates routing data from a SparseMatrix representation.
+    """
+    dispatch_indx = sparse_logits.mask_metadata.row_sorted_indx
+    combine_indx = sparse_logits.mask_metadata.col_sorted_indx
+    ragged_batch_metadata = make_ragged_tensor_metadata(
+        sparse_logits.mask_metadata.col_sum,
+        dispatch_indx.shape[0],
+    )
+    gate_scal = sparse_logits.vals.flatten()[combine_indx]
+    routing_data = RoutingData(
+        gate_scal,
+        ragged_batch_metadata.block_sizes,
+        n_expts_tot,
+        n_expts_act,
+        ragged_batch_metadata,
+    )
+    gather_idx = GatherIndx(combine_indx, dispatch_indx)
+    scatter_idx = ScatterIndx(dispatch_indx, combine_indx)
+    return routing_data, gather_idx, scatter_idx
+
+
 def legacy_routing(
     logits: torch.Tensor,
     n_expts_act: int,
@@ -158,10 +185,8 @@ def legacy_routing(
     if sm_first:
         logits = torch.softmax(logits, dim=-1)
     sparse_logits = topk(logits, n_expts_act, apply_softmax=not sm_first)
-    return legacy_routing_from_bitmatrix(
-        sparse_logits.mask,
-        sparse_logits.vals,
-        sparse_logits.indx,
+    return legacy_routing_from_sparsematrix(
+        sparse_logits,
         logits.shape[-1],
         n_expts_act,
     )