Chore export impr

Context

This MR brings some changes to allow export fallback as well as some minor fixes.
This MR is required by this aidge_export_esp32 MR.

Changes

Features

• The scheduler_export() function now only sets the backend to the nodes which do not already have one;
• It is now possible to specify a template path when using the generate_input_file() function;
• The ExportLib static files paths can now be relative ("../path").

Fix

• Changing the ExportNode.kernels_path (specify the path where the kernels are exported) had no effect.