[Enhancement] Revise SWD metric and DCGAN's configs

[LeoXing1996]

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Motivation

1. SWD metric does not handle gray-scale inputs
2. SWD metric use fake_imgs to control the number of images as input. If the passed fake_imgs is larger than length of the dataset, an error will occur. Therefore we support set fake_imgs=-1 to automatically use all images in dataset.
3. Configs of DCGAN have bugs.

Modification

Revise SWD, GenMetric and DCGAN configs.

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.

[codecov]

Codecov Report

Base: 88.84% // Head: 88.91% // Increases project coverage by +0.06% 🎉

Coverage data is based on head (50e842b) compared to base (c9ef99b).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@             Coverage Diff             @@
##           dev-1.x    #1528      +/-   ##
===========================================
+ Coverage    88.84%   88.91%   +0.06%     
===========================================
  Files          394      399       +5     
  Lines        25596    26000     +404     
  Branches      3992     4040      +48     
===========================================
+ Hits         22742    23118     +376     
- Misses        1967     1981      +14     
- Partials       887      901      +14     

Flag	Coverage Δ
unittests	88.91% <100.00%> (+0.06%)	⬆️

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

Impacted Files	Coverage Δ
mmedit/evaluation/metrics/base_gen_metric.py	88.70% <100.00%> (+0.27%)	⬆️
mmedit/evaluation/metrics/swd.py	92.66% <100.00%> (+1.57%)	⬆️
mmedit/datasets/transforms/random_degradations.py	92.25% <0.00%> (-1.77%)	⬇️
mmedit/models/editors/__init__.py	100.00% <0.00%> (ø)
mmedit/models/editors/swinir/swinir_net.py	93.38% <0.00%> (ø)
mmedit/models/editors/swinir/__init__.py	100.00% <0.00%> (ø)
mmedit/models/editors/swinir/swinir_utils.py	89.28% <0.00%> (ø)
mmedit/models/editors/swinir/swinir_rstb.py	93.78% <0.00%> (ø)
mmedit/models/editors/swinir/swinir_modules.py	96.61% <0.00%> (ø)

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