Fix Zamba2MambaMixer ignoring use_mamba_kernels=False

[sergiopaniego]

What does this PR do?

Zamba2MambaMixer.__init__ calls lazy_load_kernel("mamba-ssm") and lazy_load_kernel("causal-conv1d") unconditionally. Models that inherit from it (like NemotronH) and set use_mamba_kernels=False in their config have the flag ignored, causing failures when the kernels package is installed but causal-conv1d CUDA kernels are not available.

Fix: Gate the lazy_load_kernel calls behind getattr(config, "use_mamba_kernels", True) in the Zamba2 modular.

Related to: huggingface/trl#5278

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Who can review?

@ArthurZucker

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[github-actions]

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

run-slow: nemotron_h, zamba2