add FGD

[yzd-v]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

Add FGD(CVPR2022) for MMRazor

Modification

Add fgd config and fgd loss.

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:

• 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:

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

[codecov]

Codecov Report

❌ Patch coverage is 15.00000% with 102 lines in your changes missing coverage. Please review.
✅ Project coverage is 61.14%. Comparing base (81e0e34) to head (105489c).
⚠️ Report is 14 commits behind head on dev_v0.3.0.

Files with missing lines	Patch %	Lines
mmrazor/models/losses/fgd.py	13.55%	102 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##           dev_v0.3.0     #110      +/-   ##
==============================================
+ Coverage       58.46%   61.14%   +2.67%     
==============================================
  Files              87       91       +4     
  Lines            3154     3361     +207     
  Branches          581      602      +21     
==============================================
+ Hits             1844     2055     +211     
+ Misses           1223     1201      -22     
- Partials           87      105      +18     

Flag	Coverage Δ
unittests	61.14% <15.00%> (+2.70%)	⬆️

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[OpenMMLab-Assistant003]

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Thank you again for your contribution❤ @yzd-v