Add ImageNet format

[yasakova-anastasia]

Motivation and context

Add ImageNet format support

How has this been tested?

Unit tests

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) 2020 Intel Corporation
#
# SPDX-License-Identifier: MIT

[coveralls]

Coverage increased (+0.04%) to 60.703% when pulling a38b10e on ay/add-imagenet into 373d94c on develop.

[zhiltsov-max]

The bindings are OK, but there is an issue in formats:

• imagenet collects all labels form all annotations on an image. This line can fail, if an annotation does not have .label field. Txt is correct.

The behaviour is expected, however. I think, we probably should add a transform for collecting all labels, or a converter parameter to do this. Such a transform should be run before exporting.