[Feature] support mscoco dataset

[plyfager]

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Motivation

Please describe the motivation of this PR and the goal you want to achieve through this PR.

Modification

Support mscoco dataset for image-to-text task.
https://cocodataset.org/#home
Refer to https://github.com/microsoft/VQ-Diffusion/blob/main/image_synthesis/data/mscoco_dataset.py

Who can help? @ them here!

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
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

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects.
• CLA has been signed and all committers have signed the CLA in this PR.

[CLAassistant]

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[codecov]

Codecov Report

Base: 88.01% // Head: 87.96% // Decreases project coverage by -0.05% ⚠️

Coverage data is based on head (d735964) compared to base (112a226).
Patch coverage: 41.66% of modified lines in pull request are covered.

❗ Current head d735964 differs from pull request most recent head 8c1325f. Consider uploading reports for the commit 8c1325f to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #1520      +/-   ##
===========================================
- Coverage    88.01%   87.96%   -0.06%     
===========================================
  Files          383      384       +1     
  Lines        24183    24212      +29     
  Branches      3794     3797       +3     
===========================================
+ Hits         21285    21298      +13     
- Misses        2082     2098      +16     
  Partials       816      816              

Flag	Coverage Δ
unittests	87.96% <41.66%> (-0.06%)	⬇️

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

Impacted Files	Coverage Δ
mmedit/datasets/mscoco_dataset.py	40.00% <40.00%> (ø)
mmedit/datasets/__init__.py	100.00% <100.00%> (ø)
mmedit/models/editors/disco_diffusion/guider.py	17.35% <0.00%> (-0.35%)	⬇️

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