Op Registry upgrade

[jurevreca12]

This PR makes it possible to add QONNX Ops from outside the brevitas/finn/qonnx repositories. It adds a function register_custom_domain .

This is an example of usage of this functionality in chisel4ml:
- https://github.com/cs-jsi/chisel4ml/blob/auto_parameters/chisel4ml/preprocess/fft_torch_op.py
- https://github.com/cs-jsi/chisel4ml/blob/auto_parameters/chisel4ml/preprocess/fft_qonnx_op.py

It defines a custom pytorch op, that then gets mapped to a (Q)ONNX op.

This PR should not affect other peoples code.

[jurevreca12]

I just realized I forgot to add the example code where register_custom_domain function is used. The examples above define the ops. However, first the register_custom_domain is called https://github.com/cs-jsi/chisel4ml/blob/auto_parameters/chisel4ml/transform.py#L87, and the ops are imported into the "chisel4ml" namespace inside chise4ml/__init__.py (because QONNX equates the domain name with the python namespace).

[jmitrevs]

Can maybe more documentation be added?