Enable generation tests

[kundaMwiza]

What does this PR do?

• update generation tests structure to mirror transformers 4.29.2
• update generation strategies in optimum/graphcore/generation/utils.py to support multiple eos token to mirror transformers 4.29.2
• add destroy method to mirror poptorch_model.destroy()

Fixes # (issue)

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[katalinic-gc]

Thanks!