Add LFW format

[sizov-kirill]

Motivation and context

Added LFW format.

How has this been tested?

Checklist

• I submit my changes into the develop branch
• I have added description of my changes into CHANGELOG file
• I have updated the documentation accordingly
• I have added tests to cover my changes
• I have linked related issues (read github docs)
• I have increased versions of npm packages if it is necessary (cvat-canvas,
  cvat-core, cvat-data and cvat-ui)

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]

Will you add the format documentation file in Datumaro?

[sizov-kirill]

Will you add the format documentation file in Datumaro?

Yes, I'll add

[nmanovic]

Isn't the file optional now?

[sizov-kirill]

Yes, it's. Added clarification about it.

[nmanovic]

The main question for me now: should we allow to export tasks in the format, if they don't correspond to the expected format. For example, our tasks don't have corresponding attributes. I have created a task with pets (cat, dog) without necessary attributes and exported the dataset in LFW. Did I get something useful?

[sizov-kirill]

The main question for me now: should we allow to export tasks in the format, if they don't correspond to the expected format. For example, our tasks don't have corresponding attributes. I have created a task with pets (cat, dog) without necessary attributes and exported the dataset in LFW. Did I get something useful?

@nmanovic I think, without attributes this format can still be useful for image/face classification task.