[Feature] Add CrowdHumanDataset and Metric

[yuyoujiang]

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Motivation

I was chosen for the 1st GLCC project in https://github.com/open-mmlab/mmdetection/issues/8029, and I'm going to cooperate with @RangiLyu add CrowdHuman and CrowdDet to mmdetection.

Modification

Create crowdhuman.py to support the CrowdHuman dataset.

[CLAassistant]

All committers have signed the CLA.

[OpenMMLab-Assistant005]

Hi @yuyoujiang ！First of all, we want to express our gratitude for your significant PR in the MMDet project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

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