Use XNNPACK for stories110M model (#2895)

Summary:
Pull Request resolved: #2895

It is using portable ops. We should just advertise the XNNPACK path.

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Reviewed By: guangy10, cbilgin

Differential Revision: D55827951

fbshipit-source-id: 318f76b0fa047f9dd4ef621b1f8bb8e3af529ba0

Author: mergennachin

examples/models/llama2/README.md

@@ -81,7 +81,7 @@ If you want to deploy and run a smaller model for educational purposes. From `ex
     ```
 3. Export model and generate `.pte` file.
     ```
-    python -m examples.models.llama2.export_llama -c stories110M.pt -p params.json
+    python -m examples.models.llama2.export_llama -c stories110M.pt -p params.json -X
     ```
 4. Create tokenizer.bin.