Improve formats: COCO, Open Images, LFW

[sizov-kirill]

Summary

LFW:

• Made the file people.txt optional.
• The location of the annotation files is not specified in the original dataset. So I added the annotations directory as in formats coco, open_images, mot etc.

COCO:

• Added the opportunity to have _ character in subset names.
• Allowed to have any annotation file name when only one task is selected.

Open Images:

• Added the opportunity to import dataset without the image_ids_and_rotations.csv file.
• Added reading classes name from class-description-boxable.csv file.

How to test

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues

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) 2021 Intel Corporation
#
# SPDX-License-Identifier: MIT

[zhiltsov-max]

Ok, will you update format docs?

[sizov-kirill]

Ok, will you update format docs?

Yes, I updated documentation for COCO and Open Images. Documentation for LFW format I'll add in other PR.