Can I get COCO-style json output file on COCO validation set (or test-dev)? If so, how?

## 📚 Documentation Issue

This issue category is for problems about existing documentation, not for asking how-to questions.

* Provide a link to an existing documentation/comment/tutorial:

* How should the above documentation/comment/tutorial improve:

The issue provides no documentation link, file, test, or entry point to start from. First identify the Detectron2 evaluation entry point and existing COCO validation or test-dev documentation; done would be a clear documented procedure for producing COCO-style JSON output.

Written by the indexing model from the issue text.