Add __repr__ for Dataset

[sooahleex]

Summary

Added repr for Dataset.

• Dataset size
• source_path
• media_type
• annotated_count
• subset_count
• categories_count

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

[sooahleex]

It worked for

dm.Dataset.import_from('./tests/assets/voc_dataset/voc_dataset1', format='voc')

as

Dataset
	size=2
	source_path=./tests/assets/voc_dataset/voc_dataset1
	media_type=<class 'datumaro.components.media.Image'>
	annotated_count=1
subsets
	test: # of items=1, # of annotations=0, annotation types=[]
	train: # of items=1, # of annotations=1, annotation types=['mask', 'bbox', 'label']
categories
	label: ['background', 'aeroplane', 'bicycle', 'bird', 'boat', 'bottle', 'bus', 'car', 'cat', 'chair', 'cow', 'diningtable', 'dog', 'horse', 'motorbike', 'person', 'pottedplant', 'sheep', 'sofa', 'train', 'tvmonitor', 'ignored', 'head', 'hand', 'foot']
	mask: []

[vinnamkim]

LGTM.