[Feature (WIP)] Add Fgd

[HIT-cwh]

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Motivation

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Modification

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BC-breaking (Optional)

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Checklist

Before PR:

• Pre-commit or other linting tools are used to fix the potential lint issues.
• Bug fixes are fully covered by unit tests, the case that causes the bug should be added in the unit tests.
• The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
• The documentation has been modified accordingly, like docstring or example tutorials.

After PR:

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• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Base: 66.45% // Head: 67.55% // Increases project coverage by +1.10% 🎉

Coverage data is based on head (3d47f16) compared to base (892af92).
Patch coverage: 98.30% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
+ Coverage   66.45%   67.55%   +1.10%     
==========================================
  Files          93       94       +1     
  Lines        3467     3585     +118     
  Branches      625      631       +6     
==========================================
+ Hits         2304     2422     +118     
+ Misses       1058     1057       -1     
- Partials      105      106       +1     

Flag	Coverage Δ
unittests	67.53% <98.30%> (+1.10%)	⬆️

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

Impacted Files	Coverage Δ
mmrazor/models/losses/fgd.py	98.29% <98.29%> (ø)
mmrazor/models/losses/__init__.py	100.00% <100.00%> (ø)
mmrazor/models/algorithms/base.py	67.64% <0.00%> (+1.96%)	⬆️

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