Fix FC backward

include/aidge/backend/cpu/operator/FCImpl_kernels.hpp

@@ -136,15 +136,13 @@ void FCImpl_cpu_backward_kernel(const DimSize_t batchSize,
 
 
     // bias grad
-    if (biasesGrad == nullptr) { // no bias
-        std::fill(biasesGrad, biasesGrad + outputFeatureSize, B(0));
-    } else {
+    if (biasesGrad != nullptr) {
         for (std::size_t o = 0; o < outputFeatureSize; ++o) { // nb outputs
             B sum{0};
             for (std::size_t b = 0; b < batchSize; ++b) {
                 sum += input[b*outputFeatureSize + o];
             }
-            biasesGrad[o] = sum;
+            biasesGrad[o]+= sum;
         }
     }
 
@@ -155,7 +153,7 @@ void FCImpl_cpu_backward_kernel(const DimSize_t batchSize,
             for (std::size_t b = 0; b < batchSize; ++b) {
                 sum += originalInput[b*inputFeatureSize + c]*input[b*outputFeatureSize + o];
             }
-            weightGrad[o*inputFeatureSize + c] = sum;
+            weightGrad[o*inputFeatureSize + c]+= sum;
         }
     }
 
@@ -166,7 +164,7 @@ void FCImpl_cpu_backward_kernel(const DimSize_t batchSize,
             for (std::size_t o = 0; o < outputFeatureSize; ++o) {
                 sum += weight[o*inputFeatureSize + c] * input[b*outputFeatureSize + o];
             }
-            output[b*inputFeatureSize + c] = sum;
+            output[b*inputFeatureSize + c]+= sum;
         }
     }
 }