Add onnx support

[vishalbollu]

Closes #178, #181

• docs (round 1)
• Add pre/postprocessing for tf_api/api.py
• CLI validations for pre/postprocessor existence for TF and ONNX
• Cast python types to tensor automatically to make pre/postprocessing optional
• Validate function signature for pre/post processing functions
• Merge api.py? (Share code between api.py #202)
• Autodetect model type (Auto-detect model formats #206)

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Checklist:

• Run make test and make lint
• Test end to end manually (e.g. build/push all images, restart local operator, and run cx refresh in an example folder)
• Update documentation
• Update examples
• Alert team if dev environment changed
• Cherry-pick into release branches if it's a bugfix
• Delete the branch once it's merged

[1vn]

minor stuff