Fix task_type: change null to CAUSAL_LM string

Amp-Thread-ID: https://ampcode.com/threads/T-019c434e-990d-76bd-aab2-68b63d62e81f
Co-authored-by: Amp <amp@ampcode.com>

adapter_config.json

@@ -41,7 +41,7 @@
     "down_proj"
   ],
   "target_parameters": null,
-  "task_type": null,
+  "task_type": "CAUSAL_LM",
   "trainable_token_indices": null,
   "use_dora": false,
   "use_qalora": false,