Fixed a bug in Concat with negative axis

src/operator/Concat.cpp

@@ -49,7 +49,9 @@ std::shared_ptr<Aidge::Operator> Aidge::Concat_Op::clone() const {
 
 void Aidge::Concat_OpImpl::forward() {
     const Concat_Op& op = dynamic_cast<const Concat_Op&>(mOp);
-    const DimSize_t axis = op.axis();
+    auto axis = op.axis();
+    const auto nbDimsInput0 = op.getInput(0)->nbDims();
+    axis = (axis < 0) ? axis + static_cast<std::int32_t>(nbDimsInput0) : axis;
 
     assert(op.getInput(0) && "missing input in Concat operator");
     for (IOIndex_t i = 1; i < mOp.nbInputs(); ++i) {