fix dllm mask on set_step (#4278)

Author: grimoire

lmdeploy/pytorch/strategies/dllm/sequence.py

@@ -200,7 +200,8 @@ def set_step(self, step: int):
         start = min(step, self.num_history_ids)
         end = self.num_history_ids
         if end > start:
-            self.history_dllm_mask[start:end] = DLLM_MASKED
+            to_change_mask = self.history_dllm_mask[start:]
+            to_change_mask[to_change_mask == DLLM_CACHED] = DLLM_UNMASKED
         super().set_step(step)