Fix CVAT format import for frame stepped tasks

[zhiltsov-max]

Motivation and context

After #1807 there was a problem: after a frame is matched, a relative id is obtained. This relative id is then used for annotations where absolute id is expected. A relative id = absolute id / step.

Affected releases: 1.1.0-beta, 1.1.0

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