[Feature] Add visdrone dataset

[cydiachen]

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Motivation

Support visdrone dataset. We provide both dataset conversion script and dataset definition code.

Modification

Support visdrone dataset.

BC-breaking (Optional)

Does the modification introduce changes that break the backward compatibility of the downstream repos? If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here and update the documentation.

Checklist

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

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[cydiachen]

TODO:

1. Not yet tested on Visdrone, need to debug;
2. Visdrone has inherent label for crowd object and occluded objects, we should consider imtroduces these properties;
3. Dataset definition of dataset in MMYOLO.

[hhaAndroid]

@cydiachen

[hhaAndroid]

@cydiachen Hi. I have found other community members to push this pr forward. Thanks a lot for your contribution.