[executorch] Preprocess export test

[lucylq]

Add python e2e test exporting preprocess.

On top of: https://github.com/pytorch/executorch/pull/4603

python -m unittest  examples/models/flamingo/test_preprocess.py 

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/4651

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As of commit 4d36cf13609060d9562917e94956a547608fcbe3 with merge base 99e1ae1f5a5e8f8ef81c2a36a492c053128b75fb (): :green_heart: Looks good so far! There are no failures yet. :green_heart:

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