[Bugfix][CPU] Fix MRoPE dispatch on the CPU backend (#27800)

Signed-off-by: jiang1.li <jiang1.li@intel.com>

vllm/model_executor/layers/rotary_embedding/mrope.py

@@ -357,6 +357,15 @@ class MRotaryEmbedding(RotaryEmbeddingBase):
         key = torch.cat((key_rot, key_pass), dim=-1).reshape(key_shape)
         return query, key
 
+    def forward_cpu(
+        self,
+        positions: torch.Tensor,
+        query: torch.Tensor,
+        key: torch.Tensor | None = None,
+        offsets: torch.Tensor | None = None,
+    ) -> tuple[torch.Tensor, torch.Tensor | None]:
+        return self.forward_native(positions, query, key, offsets)
+
     @staticmethod
     def get_next_input_positions(
         mrope_position_delta: int,