[Collision] CompositeCollisionPipeline: add bwdInit for subcollision pipelines

[fredroy]

Useful for doing stuff after the init stage

---

By submitting this pull request, I acknowledge that
I have read, understand, and agree SOFA Developer Certificate of Origin (DCO).

---

Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[damienmarchal]

To me bwInit is just a deprecated design I thought we wanted to remove that.