Support Constant nodes in pytorch parser

[JanFSchulte]

Fixes https://github.com/fastmachinelearning/hls4ml/issues/1082.

Constants, for example for multiplications, are recognized and added to the hls4ml model using the Constant layer introduced in the QONNX parser. Thanks to @sei-jgwohlbier for finding the issue and designing the fix.

Type of change

For a new feature or function, please create an issue first to discuss it with us before submitting a pull request.

Note: Please delete options that are not relevant.

• [x] Bug fix (non-breaking change that fixes an issue)
• [x] New feature (non-breaking change which adds functionality)

Tests

Checklist

• [x] I have read the guidelines for contributing.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [] 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.
• [] I have added tests that prove my fix is effective or that my feature works.