[SegGPT] Fix seggpt image processor
Author: EduardoPach
Pull request: yes

# What does this PR do?

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Fixes https://github.com/huggingface/transformers/issues/29549 and add appropriate tests to avoid this issue to repeat itself in the future


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c.c. @amyeroberts 

Comments:
The fix is probably not ideal (the current way with try..except), but just wanted to start the conversation with a working solution

c.c. @NielsRogge