[Kernel] Route large SM90 row/column-scaled FP8 GEMMs to Torch (#34318)

test/registered/kernels/ops/layernorm/test_kernels_namespace.py

@@ -25,7 +25,7 @@ EXPECTED = {
     "activation.relu2": {"jit", "torch", "torch_compile"},
     "layernorm.rmsnorm": {"aot", "jit", "aiter", "torch_npu", "torch", "torch_compile"},
     "layernorm.gemma_rmsnorm": {"aot", "jit", "torch_npu", "torch", "torch_compile"},
-    "gemm.fp8_scaled_mm": {"aot"},
+    "gemm.fp8_scaled_mm": {"aot", "torch", "torch_compile"},
     "moe.moe_align_block_size": {"aot", "jit"},
     "quantization.nvfp4_gemm_swiglu_nvfp4_quant": {"cute_dsl"},
     "kvcache.reshape_and_cache_flash": {"triton"},
@@ -84,7 +84,20 @@ def test_sparse_linear_attention_registry_targets_forward_kernel():
 
 
 def test_single_backend_resolves_without_backend():
-    assert K.select_kernel("gemm.fp8_scaled_mm").backend is KernelBackend.AOT
+    assert (
+        K.select_kernel("kvcache.reshape_and_cache_flash").backend
+        is KernelBackend.TRITON
+    )
+
+
+def test_fp8_scaled_mm_requires_explicit_registry_backend(monkeypatch):
+    monkeypatch.setattr(sel, "_platform", lambda: _SM90)
+    with pytest.raises(ValueError, match="multiple backends"):
+        K.select_kernel("gemm.fp8_scaled_mm")
+    assert (
+        K.select_kernel("gemm.fp8_scaled_mm", backend=KernelBackend.AOT).backend
+        is KernelBackend.AOT
+    )
 
 
 def test_unknown_op_or_backend_raises():