new typing errors from flake8 #4964
Description
Describe the bug
new typing errors from flake8 for example
https://github.com/Project-MONAI/MONAI/runs/7957404786?check_suite_focus=true
/home/runner/work/MONAI/MONAI/monai/fl/utils/filters.py:17:1: Filter is an abstract base class, but it has no abstract methods. Remember to use @abstractmethod, @abstractclassmethod and/or @abstractproperty decorators.
1 Filter is an abstract base class, but it has no abstract methods. Remember to use @abstractmethod, @abstractclassmethod and/or @abstractproperty decorators.
1
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Please run auto style fixes: ./runtests.sh --autofix