[#11455][fix] Fallback to triton_ssm for nvfp4 quantization (#11456)

Signed-off-by: Gal Hubara Agam <96368689+galagam@users.noreply.github.com>

tensorrt_llm/_torch/auto_deploy/transform/library/ssm_cache.py

@@ -1,6 +1,12 @@
 """A set of transforms to handle SSM cache transforms."""
 
-from ..interface import TransformRegistry
+from typing import Tuple
+
+from torch.fx import GraphModule
+
+from ...models.factory import ModelFactory
+from ...shim.interface import CachedSequenceInterface
+from ..interface import SharedConfig, TransformInfo, TransformRegistry
 from .kvcache import _InsertCachedOperator
 
 
@@ -9,6 +15,23 @@ from .kvcache import _InsertCachedOperator
 class SSMCacheTransform(_InsertCachedOperator):
     """A transform to handle SSM cache operations."""
 
+    def _apply(
+        self,
+        gm: GraphModule,
+        cm: CachedSequenceInterface,
+        factory: ModelFactory,
+        shared_config: SharedConfig,
+    ) -> Tuple[GraphModule, TransformInfo]:
+        qcfg = factory.get_quant_config()
+        is_nvfp4 = qcfg.get("quant_algo", "").upper() == "NVFP4"
+        if is_nvfp4 and self.config.backend == "flashinfer_ssm":
+            self._log_warning(
+                f"SSM backend '{self.config.backend}' is not compatible with NVFP4 quantization. "
+                f"Falling back to triton_ssm."
+            )
+            self.config.backend = "triton_ssm"
+        return super()._apply(gm, cm, factory, shared_config)
+
 
 @TransformRegistry.register("insert_cached_causal_conv")
 class InitializeCausalConvCache(_InsertCachedOperator):