[one-cmds] Refactor onnx_legalizer

[binarman]

Problem

onnx_legalizer.py code is hard to understand, need to improve it's readability.

What to do

• [x] replace general transformer.make_node method with specialized methods, like make_gemm #8202
• [x] add '_' to hide internal functions #8419
• [x] add more comments to main functions #8405
• [ ] add helper methods to change output tensor in operations (need them to remove reset of make_node calls)
• [x] Remove unused variables #8419

[seanshpark]

Title typo?