Vectorize load instructions in dmmv f16 CUDA kernel (llama/9816)

Vectorize load instructions in dmmv f16 CUDA kernel (llama/9816)

* Vectorize load instructions in dmmv f16 CUDA kernel

Replaces scalar with vector load instructions, which substantially
improves performance on NVIDIA HBM GPUs, e.g. gives a 1.27X overall
speedup for Meta-Llama-3-8B-Instruct-F16 BS1 inference evaluation on
H100 SXM 80GB HBM3. On GDDR GPUs, there is a slight (1.01X) speedup.

* addressed comment

* Update ggml/src/ggml-cuda/dmmv.cu

Co-authored-by: Johannes Gäßler <redacted>
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Co-authored-by: Johannes Gäßler <redacted>