imatrix : fix crash when using --show-statistics with zero counts (#19532)

* imatrix: fix crash when using --show-statistics with zero counts

Fixes division by zero that caused floating point exceptions when processing imatrix files with zero count values. Added checks to skip zero counts and handle empty activation vectors.

Fix for the bug #19190

* imatrix: lower log level for zero-count skip message to DBG

tools/imatrix/imatrix.cpp

@@ -143,11 +143,20 @@ static void compute_statistics(std::vector<tensor_statistics> & tstats, const st
     activations.reserve(e.values.size());
 
     for (int i = 0; i < n_mat; ++i) {
+        if (e.counts[i] == 0) {
+            LOG_DBG("%s: skipping tensor %s due to zero count at index %d\n", __func__, name.c_str(), i);
+            continue;
+        }
         for (int j = 0; j < row_size; ++j) {
             activations.push_back(e.values[i*row_size + j] / e.counts[i]);
         }
     }
 
+    if (activations.empty()) {
+        LOG_ERR("%s: all counts are zero for tensor %s, skipping statistics computation\n", __func__, name.c_str());
+        return;
+    }
+
     const float act_total     = std::accumulate(activations.begin(), activations.end(), 0.0f);
     const float act_max       = *std::max_element(activations.begin(), activations.end());
     const float act_min       = *std::min_element(activations.begin(), activations.end());
@@ -1142,10 +1151,12 @@ static bool show_statistics(const common_params & params) {
             blk = -1;  // not a block layer
         }
 
+        const float entropy_norm = (tstat.elements > 0) ? 100.0f * (tstat.entropy / std::log2(tstat.elements)) : 0.0f;
+
         LOG_INF("%5s\t%-20s\t%10.2f\t%8.4f\t%11.4f\t%6.2f\t%6.2f\t%8.2f%%\t%6d\t%10.4f\t%6.2f%%\t%10.2f%%\t%8.4f\n",
                 layer.c_str(), name.c_str(), tstat.total_sqract, tstat.min_sqract, tstat.max_sqract, tstat.mean_sqract,
                 tstat.stddev, tstat.active * 100.0f, tstat.elements, tstat.entropy,
-                100.0f * (tstat.entropy / std::log2(tstat.elements)), 100.0f * tstat.zd, tstat.cossim);
+                entropy_norm, 100.0f * tstat.zd, tstat.cossim);
 
         const float weighted_bias   = tstat.elements * tstat.total_sqract;
         const float weighted_zd     = tstat.elements * tstat.zd;