Export pre-trained model using ONNX

[btreeby]

Explore exporting the pre-trained model using the Open Neural Network Exchange (ONNX) format. This could then be imported into, e.g., MATLAB so the model can be directly run. One possible complication is the use of the FFT within the residual calculation, which may not be supported.

[astanziola]

Should be OK to export modules with FFTs: onnx/onnx#3741