[CPU] Support FP8 KV cache

[bigPYJ1151]

This PR added FP8-E5M2 KV cache support for the CPU backend, and also updated tests for chunked-prefill with Half dtype.

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[Isotr0py]

The added fp8 tests passed on my side locally, LGTM!

[Isotr0py]

Seems that chunked_prefill/prefix_caching doesn't work with fp8 kv_cache, perhaps add a check with better error message here?

[rank0]:   File "/data/develop-projects/github-repos/vllm-cpu/vllm/attention/backends/torch_sdpa.py", line 544, in forward
[rank0]:     ipex_modules.PagedAttention.flash_attn_varlen_func(
[rank0]:   File "/data/develop-projects/github-repos/vllm-cpu/.venv/lib/python3.12/site-packages/intel_extension_for_pytorch/llm/modules/mha_fusion.py", line 622, in flash_attn_varlen_func
[rank0]:     ).flash_attn_varlen_func(
[rank0]:       ^^^^^^^^^^^^^^^^^^^^^^^
[rank0]:   File "/data/develop-projects/github-repos/vllm-cpu/.venv/lib/python3.12/site-packages/intel_extension_for_pytorch/transformers/models/cpu/fusions/mha_fusion.py", line 415, in flash_attn_varlen_func
[rank0]:     torch.ops.torch_ipex.flash_attn_varlen_func(
[rank0]:   File "/data/develop-projects/github-repos/vllm-cpu/.venv/lib/python3.12/site-packages/torch/_ops.py", line 1123, in __call__
[rank0]:     return self._op(*args, **(kwargs or {}))
[rank0]:            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
[rank0]: RuntimeError: expected scalar type BFloat16 but found Float8_e5m2
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