Dataset meta file for some formats

[yasakova-anastasia]

Summary

Added meta file support in following formats:

• ADE20K2017
• ADE20K2020
• Align CelebA
• CelebA
• CIFAR
• KITTI
• MNIST
• MNIST CSV
• SYNTHIA
• YOLO

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]

Suggested change

	To add unformatted classes, you can use `dataset_meta.json`.
	Information about `dataset_meta.json` can be found [here](/docs/user_manual/supported_formats/#dataset-meta-file).
	To add unformatted classes, you can use [`dataset_meta.json`](/docs/user_manual/supported_formats/#dataset-meta-file).

Please find a better term for "unformatted classes", its meaning is not clear enough.

[yasakova-anastasia]

Custom classes?

[zhiltsov-max]

Yes, it works for me,