Prepend `bos token` to Blip generations (#29642)
* prepend "bos" to blip generation
* minor changes
* Update src/transformers/models/blip_2/modeling_blip_2.py
Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>
* Update src/transformers/models/instructblip/modeling_instructblip.py
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
* add generation tester mixin
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Co-authored-by: Joao Gante <joaofranciscocardosogante@gmail.com>
Co-authored-by: amyeroberts <22614925+amyeroberts@users.noreply.github.com>
