[Enhance] Substitute the environment variable in config file.

[jbwang1997]

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Motivation

Enable using environment variables in config files. Can make config more flexible.

Example 1

demo_config.py

# dataset settings
dataset_type = 'CocoDataset'
data_root = '{{$MMDET_DATASET:/data/coco/}}'

# file_client_args = dict(
#     backend='petrel',
#     path_mapping=dict({
#         './data/': 's3://openmmlab/datasets/detection/',
#         'data/': 's3://openmmlab/datasets/detection/'
#     }))
file_client_args = dict(backend='disk')

test.py

from mmengine.config import Config

cfg = Config.fromfile('demo_config.py')
print(cfg)

The results of this pr are as below:

>> python test.py
Config (path: demo_config.py): {'dataset_type': 'CocoDataset', 'data_root': '/data/coco/', 'file_client_args': {'backend': 'disk'}}
>> MMDET_DATASET=/home/my_dataset/ python test.py
Config (path: demo_config.py): {'dataset_type': 'CocoDataset', 'data_root': '/home/my_dataset/', 'file_client_args': {'backend': 'disk'}}

Example 2

demo_config.py

model=dict(
    bbox_head=dict(
        num_classes={{'$NUM_CLASSES:80'}}))

The results of this pr are as below:

>> python test.py
Config (path: demo_config.py): {'model': {'bbox_head': {'num_classes': 80}}}
>> NUM_CLASSES=20 python test.py
Config (path: demo_config.py): {'model': {'bbox_head': {'num_classes': 20}}}

Modification

Please briefly describe what modification is made in this PR.

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.

[HAOCHENYE]

We should also update the config tutorial after the document refactoring. I'll make a notification when it is ready to update the config tutorial.

[codecov]

Codecov Report

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Additional details and impacted files

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[CLAassistant]

All committers have signed the CLA.

[HAOCHENYE]

Approved!!!

[jbwang1997]

Do I need to update config tutorial now or waiting for document refactoring finished?

[HAOCHENYE]

Do I need to update config tutorial now or waiting for document refactoring finished?

Document refactoring has been finished! You can update the tutorial now 😄 ~

[captainIT]

在 Windows 环境中 如果有个环境变量包含windows路径，如下：

import regex as re
regexp = '\\{\\{[\\\'\\"]?\\s*\\$WORK_DIR\\s*\\:\\s*None\\s*[\\\'\\"]?\\}\\}'
value = "D:\\666\\work_dir"
config_file='_work_dir = {{"$DLP_WORK_DIR:None"}}'
config_file = re.sub(regexp, value, config_file)

返回错误
regex._regex_core.error: bad escape \w at position 8