[SegGPT] Fix loss calculation (#30421)
* Fixed main train issues
* Added loss test
* Update src/transformers/models/seggpt/modeling_seggpt.py
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
* Added missing labels arg in SegGptModel forward
* Fixed typo
* Added slow test to test loss calculation
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Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>