Allow `bos_token_id is None` during the generation with `inputs_embeds`
Author: LZHgrla
Pull request: yes

# What does this PR do?

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Allow `bos_token_id is None` during the generation with `inputs_embeds`.

This is important for multi-modal inputs / generation for LLMs whose `bos_token_id` is `None`

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Comments:
@amyeroberts @gante 
Hi! I have added the related tests for this PR.


@amyeroberts should be ready for a final check

FYI @zucchini-nlp