fix: Only allow possitive value of variances in batchnorm

aidge_export_cpp/kernels/batchnorm.hpp

@@ -26,7 +26,14 @@ void batchnorm_forward (
     const Rescaling_T& __restrict rescaling)
 {
     for (unsigned int output = 0; output < NB_OUTPUTS; ++output) {
-        const Output_T var = sqrt(variances[output] + epsilon);
+        // If the variance is 0, we need to avoid division by 0
+        const Output_T var = epsilon;
+
+        // If the variance is negative, we need to set it to 0 to avoid a sqrt of a negative number
+        if (variances[output] > 0.0)
+        {
+            var = sqrt(variances[output] + epsilon);
+        }
 
         for (int oy = 0; oy < OUTPUTS_HEIGHT; ++oy) {
             for (int ox = 0; ox < OUTPUTS_WIDTH; ++ox) {