[Fix] CircleCI fail due to plum-dispatch v2.0.0

[C1rN09]

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Motivation

plum-dispatch bumps to version 2.0.0 yesterday, which breaks our unittests. There are 2 issues:

1. name 'xxx' is not defined

Reson: Still under investigation.

2. TypeError: unhashable type: 'list'

Reason: plum-dispatch 2.0.0 migrates to use beartype, while beartype doesn't support Python 3.9.1. This is actually a CPython error fixed in Python 3.9.2.

Modification

Restrict plum-dispatch<2.0.0 in requirements. Once fixed, will remove the version restriction.

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 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.