Add .then to gr.Examples

[apolinario]

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Is your feature request related to a problem? Please describe.
Sometimes the app requires some pre-processing tasks that happen during the interactive session with the user (e.g.: cropping an image when it's uploaded), however, when using gr.Examples with cache_examples, this should be expedited into running all the tasks that happen in the interactive session.

Describe the solution you'd like
I'd like to be able to do a .then or .success after gr.Examples, so that once the first fn runs, others run in sequence.

[abidlabs]

Thanks @apolinario agreed. I think we can just expose the caching event so that a developer could do .then, .success, cancellation, or anything else that they'd like with the event