Allow decoding b64 string without header in processing utils (#4031)
* allow decoding b64 string without headers
* install gradio-client in edittable mode
* update GH actions
* add test for decoding headerless b64
* add test for decoding headerless b64 image
* some linting
* fix test
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Co-authored-by: Abubakar Abid <abubakar@huggingface.co>