Avoid unnecessary image loading in Datumaro format on save

[zhiltsov-max]

Summary

• Fixed the problem with occasional image loading in Datumaro format

The problem is that transforms for inputs, providing no image metainfo (size etc.), can require image loading for loading size data. Unless image saving is not required, it should not be done.

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

[jihyeonyi]

It works well in the classification_split.