[Enhancement] Make forward logic more clear for GAN models

[LeoXing1996]

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Motivation

The forward function for GAN models is unclear. Therefore we modify them in this PR.

Modification

Revise the forward logic for GAN models.

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.

[zengyh1900]

Any operations for packing and unpacking datasamples should be apart from the implementation of the algorithm.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (refactor-metric-loop-and-preprocessor@799e1f8). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head d8464ff differs from pull request most recent head d829bdb. Consider uploading reports for the commit d829bdb to get more accurate results

Additional details and impacted files

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##             refactor-metric-loop-and-preprocessor    #1670   +/-   ##
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Flag	Coverage Δ
unittests	87.80% <0.00%> (?)

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[zengyh1900]

we leave the loop for EditDataSample insideSinGAN in future PR.