Updating config: `is_causal: False`

This implementation is using non-causal attention.
For future correct behavior, `is_causal=False` marker will help identify this implementation detail.

config.json

@@ -28,5 +28,6 @@
   "transformers_version": "4.41.2",
   "use_cache": true,
   "use_sliding_window": false,
+  "is_causal": false,
   "vocab_size": 151646
 }