Add MultiImageMixDataset in build_dataset

[liuyanyi]

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Motivation

For future support of Mosaic and Mixup

Modification

Please briefly describe what modification is made in this PR.

BC-breaking (Optional)

Does the modification introduce changes that break the back-compatibility of the downstream repos?
If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.

Use cases (Optional)

If this PR introduces a new feature, it is better to list some use cases here, and update the documentation.

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
3. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #331 (d1f9595) into dev (3055b56) will decrease coverage by 0.03%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##              dev     #331      +/-   ##
==========================================
- Coverage   31.59%   31.56%   -0.04%     
==========================================
  Files         113      113              
  Lines        7355     7360       +5     
  Branches     1128     1129       +1     
==========================================
- Hits         2324     2323       -1     
- Misses       4929     4933       +4     
- Partials      102      104       +2     

Flag	Coverage Δ
unittests	31.56% <16.66%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
mmrotate/datasets/builder.py	42.42% <16.66%> (-9.43%)	⬇️
mmrotate/utils/setup_env.py	88.00% <0.00%> (-4.00%)	⬇️
mmrotate/apis/inference.py	0.00% <0.00%> (ø)

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