Coreml conversion

## 🚀 Feature
Simple and streamlined conversion of custom models to coreml

## Motivation & Examples

this feature would help to deploy custom trained models on ios devices

the feature could be as simple as a saving format to coreml

## Note

We only consider adding new features if they are relevant to many users.

If you request implementation of research papers -- we only consider papers that have enough significance and prevalance in the object detection field.

We do not take requests for most projects in the `projects/` directory, because they are research code release that is mainly for other researchers to reproduce results.

"Make X faster/accurate" is not a valid feature request. "Implement a concrete feature that can make X faster/accurate" can be a valid feature request.

Instead of adding features inside detectron2,
you can implement many features by [extending detectron2](https://detectron2.readthedocs.io/tutorials/extend.html).
The [projects/](https://github.com/facebookresearch/detectron2/tree/main/projects/) directory contains many of such examples.