Deduplicate q4 quantization functions (#383)

* Deduplicate q4 quantization functions

* Use const; add basic test

* Re-enable quantization test

* Disable AVX2 flags in CI

---------

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

tests/test-quantize.c

@@ -0,0 +1,42 @@
+#include "ggml.h"
+#undef NDEBUG
+#include <assert.h>
+#include <math.h>
+
+int main(void) {
+    #define QK 32
+    float src[QK];
+    uint8_t dst[24];
+    int64_t hist[16];
+
+    for (int i = 0; i < QK; i++) {
+        src[i] = (float)(i + 1);
+    }
+
+    size_t size = ggml_quantize_q4_0(src, dst, QK, QK, QK, hist);
+    assert(size == 20);
+    float max_result = ((float *)dst)[0];
+    float max_expected = src[31] / ((1 << 3) - 1);
+    assert(max_result == max_expected);
+    for (int i = 0; i < QK; i++) {
+        uint8_t q4_result = (i % 2) ? (dst[sizeof(float) + i/2] >> 4) : (dst[sizeof(float) + i/2] & 0xF);
+        uint8_t q4_expected = roundf(src[i] / max_expected) + 8;
+        assert(q4_result == q4_expected);
+    }
+
+    size = ggml_quantize_q4_1(src, dst, QK, QK, QK, hist);
+    assert(size == 24);
+    float delta_result = ((float *)dst)[0];
+    float delta_expected = (src[31] - src[0]) / ((1 << 4) - 1);
+    assert(delta_result == delta_expected);
+    float min_result = ((float *)dst)[1];
+    float min_expected = src[0];
+    assert(min_result == min_expected);
+    for (int i = 0; i < QK; i++) {
+        uint8_t q4_result = (i % 2) ? (dst[sizeof(float)*2 + i/2] >> 4) : (dst[sizeof(float)*2 + i/2] & 0xF);
+        uint8_t q4_expected = roundf((src[i] - min_expected) / delta_expected);
+        assert(q4_result == q4_expected);
+    }
+
+    return 0;
+}