Additional cleanup of the codebase

Created by: vloncar

Description

Minor cleanups around the codebase:

• Removed unused template functions from Layer
• Moved numbers_cpp() from Layer to Writers so that no C++ code generation happens in IR
• Moved definitions of Quantizers to types.py
• Cleaned up qkeras imports (it is now a hard dependency so no need for checks if it is available)

Type of change

• Other (Specify) - Cleanup

Tests

No need for extra tests, no new features were introduced

Checklist

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