Fix zamba2 rotary embedding call when use_mem_rope is False

[echarlaix]

self.rotary_emb is always called since #39847 while only being initialized when config.use_mem_rope is True

inference failing since v5 for models config.use_mem_rope=False

AttributeError: 'Zamba2Model' object has no attribute 'rotary_emb'

https://github.com/huggingface/transformers/blob/v4.57.6/src/transformers/models/zamba2/modeling_zamba2.py#L1312-L1316

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: zamba2

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.