[Feature] RTMDet Swin / ConvNeXt

[okotaku]

Motivation

Train RTMDet with other backbones.

Result

Model	size	box AP
RTMDet-l-ConvNeXt-B	640	53.1
RTMDet-l-Swin-B	640	52.4
RTMDet-l-Swin-B-P6	1280	56.4

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMPreTrain.
4. The documentation has been modified accordingly, like docstring or example tutorials.