Warn if wrong type is given for Llama export for XNNPACK

Followup to #7775

Author: mergennachin

examples/models/llama/export_llama_lib.py

@@ -674,6 +674,12 @@ def _validate_args(args):
                 "If you need this feature, please file an issue."
             )
 
+    if args.xnnpack:
+        if args.dtype_override not in [DType.fp32, DType.fp16]:
+            raise ValueError(
+                f"XNNPACK supports either fp32 or fp16 dtypes only for now. Given {args.dtype_override}."
+            )
+
 
 def _export_llama(args) -> LLMEdgeManager:  # noqa: C901
     _validate_args(args)