Fix padding

Fix for aidge#194 (closed).

According to ONNX: pads format should be: [x1_begin, x2_begin, …, x1_end, x2_end,…], where xi_begin is the number of pad values added at the beginning of axis axes[i] and xi_end, the number of pad values added at the end of axis axes[i].

Thus the computation of output dims was wrong if one wants to conform to ONNX.