Yeah I agree with your concerns @mratsim  : 5 layers of compilers would be a nightmare to debug.  This stack can be useful in deploying a really well-tested model, but will be hard to use it for research or product development setting.

How does the compiler deal with dynamic graph/variable sized input.

ONNX translates dynamic graphs to static based on most-traversed path, so rest of the tool chain would work off of the static graph.
Source: https://research.fb.com/facebook-and-microsoft-introduce-new-open-ecosystem-for-interchangeable-ai-frameworks/ 24