Mlx improve 6bit packing perf (#20806) #127

Job	Run time
CI run decision / decide	13s
Get changed files / get-changed-files	5s
export-model-cuda-windows-artifact (mistralai, Voxtral-Mini-3B-2507, non-quantized) / linux-job	1d 0h 0m 1s
export-model-cuda-windows-artifact (nvidia, parakeet-tdt, non-quantized) / linux-job	1d 0h 0m 1s
export-model-cuda-windows-artifact (mistralai, Voxtral-Mini-3B-2507, quantized-int4-weight-only) / linux-job	1d 0h 0m 1s
export-model-cuda-windows-artifact (nvidia, parakeet-tdt, quantized-int4-weight-only) / linux-job	1d 0h 0m 1s
export-model-cuda-windows-artifact (facebook, dinov2-small-imagenet1k-1-layer, non-quantized) / linux-job	1d 0h 0m 1s
export-model-cuda-windows-artifact (mistralai, Voxtral-Mini-4B-Realtime-2602, quantized-int4-tile... / linux-job	1d 0h 0m 1s
test-model-cuda-windows-e2e	0s
	6d 0h 0m 24s

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