2-bit quantizations (#4897)
* imatrix: load
* imatrix: WIP
* imatrix: Add Q2_K quantization
* imatrix: also guard against Q2_K_S quantization without importance matrix
* imatrix: guard even more against low-bit quantization misuse
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Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>
