Fix a bug in the previous behavior when importing nested datasets in the project

[vinnamkim]

Summary

• Ticket no. 127589
• Fix a bug found in Import only import one source #1214
• In the previous Datumaro version, we could import the nested datasets in the given path. For example, if we Dataset.import_from(./some_project) for the following directory structure

./some_project/
├── dataset_1
└── dataset_2

, the imported dataset include dataset_1 and dataset_2.

• This is common pattern for the Datumaro project, so that we have to fix this.

How to test

Added an integration test for this scenario.

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

License

• I submit my code changes under the same MIT License that covers the project.
  Feel free to contact the maintainers if that's a concern.
• I have updated the license header for each file (see an example below).

# Copyright (C) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (eb44355) 80.53% compared to head (bf9e6b7) 80.53%.

❗ Current head bf9e6b7 differs from pull request most recent head e0d266b. Consider uploading reports for the commit e0d266b to get more accurate results

Files	Patch %	Lines
src/datumaro/components/merge/extractor_merger.py	69.56%	4 Missing and 3 partials ⚠️
src/datumaro/cli/commands/convert.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #1243   +/-   ##
========================================
  Coverage    80.53%   80.53%           
========================================
  Files          270      270           
  Lines        30232    30238    +6     
  Branches      5898     5903    +5     
========================================
+ Hits         24348    24353    +5     
  Misses        4506     4506           
- Partials      1378     1379    +1     

Flag	Coverage Δ
ubuntu-20.04_Python-3.8	80.52% <81.39%> (+<0.01%)	⬆️
windows-2022_Python-3.8	80.50% <81.39%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[wonjuleee]

In overall, it looks good to me, but I have a one quick question. Is this fix available only for coco and yolo formats?

[vinnamkim]

In overall, it looks good to me, but I have a one quick question. Is this fix available only for coco and yolo formats?

As we discussed, it is available only for the yolo formats. This is because importing datasets from sub-directories at once is not commonly available for all data formats.