[flamingo] Update preproc imports

[lucylq]

After torchtune changes.

Test Plan:

./install_requirements.sh --pybind xnnpack
sh examples/models/flamingo/install_requirements.sh 
python examples/models/flamingo/preprocess/export_preprocess.py
python -m unittest examples/models/flamingo/preprocess/test_preprocess.py

[pytorch-bot]

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