Test (example/llm): Refactor and add basic tests for the LLM entry-point

[nickfraser]

Create a tests which fully test the end-to-end examples, since the stability of these examples is becoming increasingly important. The idea is to put into a place a structure that can be applied to other entry-points in brevitas_examples. Incorporates #987 - Now rebased on dev.

TODO:

• Move argument parser to a separate function, to allow calling from tests
• Update main function to take parsed arguments and an input, return useful output that we want to test as output
• Create run tests for tiny versions of important models:
  • Llama
  • OPT
  • Mistral
• Create run tests on all pre-quantization and PTQ techniques on all selected tiny models:
  • --ln-affine-merge
  • --weight-equalization
  • --act-equalization layerwise
  • --act-equalization fx
  • --bias-corr
  • --act-calibration
  • --gptq
• Test --replace-mha on OPT
• Test some important datatypes:
  • Weight-only (int4)
  • Int8
  • FP8
  • MXFP8 (fails)
• Create accuracy tests for tiny versions of important models (should cover all models, datatypes and PTQ techniques):
  • Llama
  • OPT
  • Mistral
• Create tests for export flows
  • ONNX QCDQ
  • Torchscript QCDQ
  • torchmlir fails and won't fix
  • torchmlir packed weights fails and won't fix
• Added test to github actions