Importance Matrix calculation (#4861)

* imatrix: 1st version

* imatrix: WIP

* Cleanup

* Update examples/imatrix/imatrix.cpp

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

---------

Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>
Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

examples/CMakeLists.txt

@@ -36,6 +36,7 @@ else()
     add_subdirectory(lookahead)
     add_subdirectory(lookup)
     add_subdirectory(train-text-from-scratch)
+    add_subdirectory(imatrix)
     if (LLAMA_METAL)
         add_subdirectory(metal)
     endif()