Additional cleanup of the codebase

[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.

[jmitrevs]

Is this for 0.7.0 or 0.8.0?

[jmduarte]

Looks good to me and the qkeras tests pass: https://gitlab.cern.ch/fastmachinelearning/hls4ml/-/jobs/28750168

Is there anything else we should test specifically?

[vloncar]

Is this for 0.7.0 or 0.8.0?

0.7.0. It doesn't make any changes to the functionality, it just reorganizes things a little bit. so it should be safe. If this messed up some imports, we would either get circular imports (all tests fail), or failure to import qkeras stuff (qkeras tests fail). But we can see from the logs that there are no import warnings and all tests pass.