[Refactor]: Refactor DAB-DETR in MMDetection 3.0

[KeiChiTse]

Motivation

Comparing to PR #8533, this PR aims to refactor DAB-DETR in the upcoming MMDetection 3.0!

I'm working together with @jshilong, @Li-Qingyun, and @LYMDLUT to refactor the DETR-like algorithms in MMDetection 3.0, using a more lightweight and easy-to-read code style.

Details can be refered to in #8763, feel free to give your advice!

[KeiChiTse]

Short-term Plan

1. Modify code based on review.
2. ResNet 50 mAP alignment with official repo.

[KeiChiTse]

Update:

Forward results and losses for a batch are perfectly aligned (4 decimal places) with 2.0 version and official repo!

[CLAassistant]

All committers have signed the CLA.

[jshilong]

please merge the latest refactor_detr

[jshilong]

please merge the refactor_detr and resolve the conflict

[ZwwWayne]

closed as it has been completed in #9252