[Hardware][Intel GPU] add XPU bf16 support

[jikunshang]

No description provided.

[github-actions]

👋 Hi! Thank you for contributing to the vLLM project.
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🚀

[mgoin]

What changed to allow this? There are no dependency updates listed - does this require a newer version of OneAPI?

[jikunshang]

@mgoin bf16 should be ready after oneapi 2024.2, we forgot to update this.