Make safety_checker optional in more pipelines

[pcuenca]

Fixes #1792.

Note that some pipelines (paint by example, image variations) do require a feature_extractor. We should at least display a clear error message if the loaded checkpoint doesn't have one. Or should we automatically instantiate one in those cases?

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[patil-suraj]

Thanks for fixing this! Also, since the feature_extractor is a required argument for those pipelines, it should already display a nice error message about that no ? And think we can-not load feature_extractor automatically because those are associated with specific image models and might not be the same for all.

[pcuenca]

since the feature_extractor is a required argument for those pipelines, it should already display a nice error message about that no ?

This is what it gives right now:

/home/pedro/.cache/huggingface/diffusers/models--pcuenq--tmp-variations/snapshots/09d9d929c0c58a2ae7a96b7b371e9e91c834e821 does not appear to have a file named preprocessor_config.json. Checkout 'https://huggingface.co//home/pedro/.cache/huggingface/diffusers/models--pcuenq--tmp-variations/snapshots/09d9d929c0c58a2ae7a96b7b371e9e91c834e821/None' for available files.

I think we could maybe improve it in a new PR. Merging this because I think fixing the issue is more important than the error message for those other pipelines :)

[pcuenca]

Actually, taking a look at the test failures.

[pcuenca]

Tests now pass, cc @anton-l for the changes and merging to fix the issue.