[Pixtral] Improve loading

[patrickvonplaten]

Pixtral loading was very memory-inefficient before because itertool's tee operator required 3x more memory than necessary. This PR fixes it

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
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[patrickvonplaten]

cc @ywang96

[ywang96]

LGTM!