[MLA] add `merge_attn_states` sycl kernel #64

jikunshang wants to merge 3 commits intovllm-project:mainvllm-project/vllm-xpu-kernels:mainfrom

jikunshang:kunshang/mla_kernelsjikunshang/vllm-xpu-kernels:kunshang/mla_kernelsCopy head branch name to clipboard