Add flax installation in daily doctest workflow (#25860)

fix
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

.github/workflows/doctests.yml

@@ -34,7 +34,7 @@ jobs:
           nvidia-smi
 
       - name: Install transformers in edit mode
-        run: python3 -m pip install -e .
+        run: python3 -m pip install -e .[flax]
 
       - name: GPU visibility
         run: |