onnx: export model constants as outputs, instead of in the initializer list

[chriselion]

Proposed change(s)

Recent versions of barracuda don't properly read the constants from the onnx file, so the onnx files unusable for inference.

This changes the onnx export to put them in the "outputs" for the model, so we can be sure they're preserved in export and read in barracuda.

I'm working on yamato tests in parallel for this, but wanted to get the fix in now.

Useful links (Github issues, JIRA tickets, ML-Agents forum threads etc.)

Types of change(s)

• Bug fix
• New feature
• Code refactor
• Breaking change
• Documentation update
• Other (please describe)

Checklist

• Added tests that prove my fix is effective or that my feature works
• Updated the changelog (if applicable)
• Updated the documentation (if applicable)
• Updated the migration guide (if applicable)

Other comments

[ervteng]