Add tfds:uc_merced and tfds:eurosat download

[vinnamkim]

Summary

• Ticket no. 107273
• Add tfds:uc_merced and tfds:eurosat download
• These datasets will be used for the merge tutorial

How to test

Checklist

• I have added unit tests to cover my changes.​
• I have added integration tests to cover my changes.​
• I have added the description of my changes into CHANGELOG.​
• I have updated the documentation accordingly

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) 2023 Intel Corporation
#
# SPDX-License-Identifier: MIT

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01 ⚠️

Comparison is base (edfd106) 78.86% compared to head (79b3f1b) 78.85%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #914      +/-   ##
===========================================
- Coverage    78.86%   78.85%   -0.01%     
===========================================
  Files          215      215              
  Lines        25200    25202       +2     
  Branches      5043     5043              
===========================================
  Hits         19873    19873              
- Misses        4192     4193       +1     
- Partials      1135     1136       +1     

Flag	Coverage Δ
macos-11_Python-3.8	?
ubuntu-20.04_Python-3.8	78.85% <100.00%> (+<0.01%)	⬆️
windows-2019_Python-3.8	78.79% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
datumaro/components/extractor_tfds.py	95.85% <100.00%> (+0.03%)	⬆️

... and 1 file with indirect coverage changes

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