Register layer attributes in VivadoAccelerator backend

[vloncar]

Description

VivadoAccelerator backend doesn't call _register_layer_attributes() so the layers with backend-specific attributes don't get extended as with the Vivado backend. This means the layers that expect some default values don't get them, causing conversion issues. Fixes #722

Type of change

• [x] Bug fix (non-breaking change that fixes an issue)

Tests

Test from #722 should suffice.

Checklist

• [x] I have read the guidelines for contributing.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [x] I have made corresponding changes to the documentation.
• [x] My changes generate no new warnings.
• [x] I have installed and run pre-commit on the files I edited or added.
• [x] I have added tests that prove my fix is effective or that my feature works.