Support ASFormer

[jts250]

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

Motivation

Implement ASFormer.

Modification

Adding configuration files, model files (ASFormer), dataset files, and evaluation metrics for inference of video action segmentation.

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.

Notes (Optional)

This branch is to be optimized.

[codecov]

Codecov Report

Attention: 2794 lines in your changes are missing coverage. Please review.

Files	Coverage Δ
mmaction/apis/__init__.py	100.00% <ø> (ø)
...maction/apis/inferencers/actionrecog_inferencer.py	74.07% <100.00%> (ø)
mmaction/datasets/__init__.py	100.00% <100.00%> (ø)
mmaction/datasets/ava_dataset.py	86.46% <ø> (ø)
mmaction/datasets/transforms/__init__.py	100.00% <100.00%> (ø)
mmaction/datasets/transforms/processing.py	79.85% <ø> (ø)
mmaction/engine/runner/__init__.py	100.00% <100.00%> (ø)
...maction/evaluation/functional/multisports_utils.py	64.05% <100.00%> (+0.23%)	⬆️
mmaction/evaluation/metrics/__init__.py	100.00% <100.00%> (ø)
mmaction/models/__init__.py	100.00% <100.00%> (ø)
... and 66 more

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