[Refactor] Refactor random degradation

[Z-Fran]

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Motivation

Refactor random degradation to compute on the range of 0-255

Modification

Please briefly describe what modification is made in this PR.

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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)

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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.

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[codecov]

Codecov Report

Base: 87.90% // Head: 87.90% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (44239a6) compared to base (6de38d0).
Patch coverage: 76.92% of modified lines in pull request are covered.

❗ Current head 44239a6 differs from pull request most recent head 8345855. Consider uploading reports for the commit 8345855 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           dev-1.x    #1583   +/-   ##
========================================
  Coverage    87.90%   87.90%           
========================================
  Files          399      399           
  Lines        26015    26016    +1     
  Branches      4044     4044           
========================================
+ Hits         22868    22869    +1     
  Misses        2240     2240           
  Partials       907      907           

Flag	Coverage Δ
unittests	87.90% <76.92%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
mmedit/models/editors/real_esrgan/real_esrgan.py	82.14% <66.66%> (ø)
mmedit/datasets/transforms/random_degradations.py	92.25% <71.42%> (ø)
mmedit/datasets/transforms/aug_pixel.py	96.38% <100.00%> (+0.01%)	⬆️

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[zengyh1900]

lgtm.