[LLM] Fix Qwen registered_causal_mask is None (#9513)

* Add registered_causal_mask init based on 2abd8e5777.

python/llm/src/bigdl/llm/transformers/models/qwen.py

@@ -174,6 +174,9 @@ def qwen_attention_forward(
         context_layer = context_layer.flatten(2, 3).contiguous()
 
     else:
+        registered_causal_mask = torch.tril(
+            torch.ones((key.size(1), key.size(1)), dtype=torch.bool, device=key.device)
+        ).view(1, 1, key.size(1), key.size(1))
         query = query.permute(0, 2, 1, 3)
         if not self.use_cache_quantization:
             key = key.permute(0, 2, 1, 3)