Support case when cache_len == 0

Differential Revision: D90526985

Pull Request resolved: #16546

Author: YIWENX14

examples/models/llama/static_attention.py

@@ -632,6 +632,8 @@ def _run_once(
         return y, attn_updates
 
     def _update_states(self, attn_updates, update_pos, update_len):
+        if attn_updates["out_cache_state"] is None:
+            return
         for mask in self._masks.values():
             mask.unmask(update_len)
         k_cache_updates, v_cache_updates = attn_updates["out_cache_state"]