[feat] add basic webcam demo (#659)

[antileaf]

Motivation

In order to get started more quickly, a webcam demo which is easy-to-use is needed. Also solving issue #659.

Modification

Added demo/python/webcam_demo.py, demo/python/webcam_demo/__init__.py and demo/python/webcam_demo/misc.py.

BC-breaking (Optional)

There is no BC-breaking.

Use cases (Optional)

The webcam demo can be used to show how to take use of the APIs of MMDeploy. This is a basic version, and more features may be added soon.

Checklist

• [x] Pre-commit or other linting tools are used to fix the potential lint issues.
• [x] The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
• [x] If the modification has a dependency on downstream projects of a newer version, this PR should be tested with all supported versions of downstream projects.
• [ ] The documentation has been modified accordingly, like docstring or example tutorials.

[lvhan028]

Please merge master to resolve readthedocs build error

[irexyc]

I think the file structure is not appropriate. Maybe we can add a folder in demo/python and move the related files to it.

It seems like it only support specific models, it should explain in readme.

The strategy of check_model is not good, we can read the meta info from detail.json and deploy.json.

An actual run command is needed. and it's better to have a demo picture.

To lower the bar of running the demo, the user guide should be more detailed. And it's better to have a link of pre-converted models

[ChenZhenGui]

hello, I want to ask how to use the models.py while my model is not the DEEPPOSE_RESNET?

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