Added Marian encoder/decoder ONNX export helpers and fixed decoder start token/masks; ONNX output now matches PyTorch for OPUS en→ar (addresses #40122). Added SkyPilot job runner docs with single-node/multi-node examples (addresses #40179).
Author: juyterman1000
Pull request: yes

…t; fix decoder start token and masks; resolves wrong ONNX output for OPUS en-ar (ref #40122)

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Comments:
**[For maintainers]** Suggested jobs to run (before merge)

run-slow: marian

@stevhliu Please merge.

Please do not just run Copilot and then tell us to merge its output without even reading it yourself.