[Feature] stdet+recognition demo

[gengenkai]

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Motivation

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Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduces changes that break the back-compatibility of this repo?
If so, please describe how it breaks the compatibility and how users should modify their codes to keep compatibility with this PR.

Use cases (Optional)

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Checklist

1. Pre-commit or other linting tools should be used to fix the potential lint issues.
2. The modification should be covered by complete unit tests. If not, please add more unit tests 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 should be modified accordingly, like docstring or example tutorials.

[dreamerlin]

Use tempfile to make temporary directory, https://github.com/open-mmlab/mmaction2/blob/master/tests/test_runtime/test_eval_hook.py#L80

[kennymckormick]

Note that:

1. You should perform inference only when the user wants you to do so.
2. The visualization should be more powerful: what if one need to visualize four prediction results at the same time?