Motivation

Sometimes, we want to create a bash script with some default --cfg-options, like

python tools/train.py $1 --cfg-options default_hooks.checkpoint.save_best=True ${@:2}

but when we use the script, and pass another --cfg-options to it, the last one will overwrite the previous completely.

Modification

According to the extend action of Python, I modified the DictAction in MMEngine, and now it supports multiple --cfg-options and will concat these options.

BC-breaking (Optional)

Almost not, unless users exactly use multiple options and want the last one to overwrite the previous one.

Use cases (Optional)

python tools/train.py $1 --cfg-options default_hooks.checkpoint.save_best=True --cfg-options train_cfg.max_epochs=300

And we will get both options.

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.