Add notebook testing to nightly build job (#785)

# What does this PR do?

Adds testing of the notebook to the nightly build job 

## Test Plan

Here is a sample run -- 

1281588919

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Co-authored-by: Hardik Shah <hjshah@fb.com>

.github/workflows/publish-to-test-pypi.yml

@@ -199,7 +199,13 @@ jobs:
       - publish-to-testpypi
       - trigger-client-and-models-build
     runs-on: ubuntu-latest
+    env:
+      TOGETHER_API_KEY: ${{ secrets.TOGETHER_API_KEY }}
+      TAVILY_SEARCH_API_KEY: ${{ secrets.TAVILY_SEARCH_API_KEY }}
     steps:
+    - uses: actions/checkout@v4
+      with:
+        persist-credentials: false
     - name: Install the package
       run: |
         max_attempts=6
@@ -228,5 +234,10 @@ jobs:
         llama stack list-apis
         llama stack list-providers inference
         llama stack list-providers telemetry
+    - name: Test Notebook
+      run: |
+        pip install pytest nbval
+        llama stack build --template together --image-type venv
+        pytest -v -s --nbval-lax ./docs/notebooks/Llama_Stack_Building_AI_Applications.ipynb
 
     # TODO: add trigger for integration test workflow & docker builds