Add ONNX / ONNXRuntime support for StarCoder (#1042)
* add gpt-bigcode
* add bigcode specific dummy generator
* normalize config
* test past key value flattened
* revert past key value separation
* fix
* add test to exporter
* add ort modeling support
* add modeling test
* fix bloom
* fix typo
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Co-authored-by: JingyaHuang <jingya@huggingface.co>
