Description

Support to set data root through commands like export MMDET_DATASETS=/path/to/datasets, in this way we do not need to modify the config or set up soft links.

Steps

• Implement the feature
• Add unit tests to test your code

Resources

• https://github.com/facebookresearch/detectron2/blob/main/datasets/README.md
• related issue: Is there a better way to specify data root? #6915

Reviewers

@jbwang1997