[Kernel] Unify BaseFusedOp and MultiPlatformOp dispatch (#33205)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

test/manual/kernels/bench_fused_op_dispatch.py

@@ -0,0 +1,81 @@
+"""Hot-path dispatch overhead microbenchmark for the unified ``BaseFusedOp``.
+
+Compares per-call overhead of:
+
+1. a plain bound-method call (theoretical floor),
+2. a minimal reproduction of the former ``MultiPlatformOp`` hot path
+   (``nn.Module.__call__`` -> ``self._forward_method(*args)``),
+3. the unified ``BaseFusedOp`` (adds the forced-backend check, the cached
+   dispatch lookup, and the trace flag check).
+
+The op body is a no-op so the numbers isolate pure dispatch overhead; real
+kernels are microseconds+, so the delta reported here is the worst case.
+
+Run locally (CPU is fine):
+    python test/manual/kernels/bench_fused_op_dispatch.py
+"""
+
+import time
+
+import torch
+from torch import nn
+
+from sglang.kernels.fused_op import BaseFusedOp
+
+N_WARMUP = 10_000
+N_ITERS = 200_000
+
+
+class _OldStyleOp(nn.Module):
+    """Minimal replica of the retired MultiPlatformOp hot path."""
+
+    def __init__(self):
+        super().__init__()
+        self._forward_method = self.forward_cuda
+
+    def forward(self, *args, **kwargs):
+        return self._forward_method(*args, **kwargs)
+
+    def forward_cuda(self, x):
+        return x
+
+
+class _NewOp(BaseFusedOp):
+    op = "bench.dispatch"
+
+    def forward_native(self, x):
+        return x
+
+    def forward_cuda(self, x):
+        return x
+
+
+def _bench(fn, x) -> float:
+    for _ in range(N_WARMUP):
+        fn(x)
+    start = time.perf_counter()
+    for _ in range(N_ITERS):
+        fn(x)
+    return (time.perf_counter() - start) / N_ITERS * 1e9  # ns/call
+
+
+def main():
+    x = torch.zeros(1)
+
+    old_op = _OldStyleOp()
+    new_op = _NewOp()
+    new_op(x)  # resolve + cache dispatch
+    bound = new_op.forward_native
+
+    floor_ns = _bench(bound, x)
+    old_ns = _bench(old_op, x)
+    new_ns = _bench(new_op, x)
+
+    print(f"plain bound method      : {floor_ns:8.1f} ns/call")
+    print(f"MultiPlatformOp replica : {old_ns:8.1f} ns/call")
+    print(f"unified BaseFusedOp     : {new_ns:8.1f} ns/call")
+    print(f"delta (new - old)       : {new_ns - old_ns:8.1f} ns/call")
+
+
+if __name__ == "__main__":
+    main()