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>