[Feature] Add vis backend for MLflow.

[sh0622-kim]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

I was reading the API documentation to use MLflow. With a recent update, I found out that MLflow is not available
I read that issue #731 is looking for a developer to develop the features I want. I hope this helps.

Modification

Add vis backend for MLflow.

BC-breaking (Optional)

While adding VIS and verifying it, a NoneType error occurred in mmengine/model/base_module.py Line 170: handler.stream.write, so a conditional statement was added. Please review if there is any problem.

Use cases (Optional)

visualizer = dict(
    type='ClsVisualizer', vis_backends=[dict(type='LocalVisBackend'), dict(type='MLflowVisBackend')])

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 MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[CLAassistant]

All committers have signed the CLA.

[HAOCHENYE]

BTW, we should also update docs/en/api/visualization.rst and docs/zh_cn/api/visualization.rst

[sh0622-kim]

@zhouzaida

Is there any way to verify that this code works with Multi GPU training?
I have not been able to verify it due to limited resources in my development environment.

[HAOCHENYE]

@zhouzaida

Is there any way to verify that this code works with Multi GPU training? I have not been able to verify it due to limited resources in my development environment.

Thanks for your contribution. I've tested it in ddp training and it worked as expected. We'll merge this PR ASAP!

[zhouzaida]

@HAOCHENYE Thanks for your contribution. This PR will be merged after passing the CI.

[OpenMMLab-Assistant001]

Hi @sh0622-kim ！We are grateful for your efforts in helping improve this open-source project during your personal time.

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Thank you again for your contribution❤

[zimonitrome]

Nice, now we only need the ability to log custom artifacts too.