Hotfix the default backend issue

Description

Previous policy regarding the backend management in the PTQ was to set the new nodes backends to their parent ones.

While it looks like a valid approach, it has two caveats :

• Firstly, the parent may not have any backend (like for example the Concat nodes) which causes a registrar issue.
• Secondly, the whole graphView backend is overwritten multiple times in the PTQ pipeline so the information about the original parent backend is in fact lost.

In absence of a better solution, we propose (for now) to set the new nodes backends to cpu by default, and to overwrite the whole graphView backend if useCuda is set to true.