[Feature] Support automatically scaling LR according to GPU number and samples per GPU

[humu789]

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Motivation

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Modification

Please briefly describe what modification is made in this PR.

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:

• [ ] 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.
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[codecov[bot]]

Codecov Report

Merging #131 (5eea6d3) into dev_v0.3.0 (f5ee768) will decrease coverage by 0.58%. The diff coverage is 15.78%.

@@              Coverage Diff               @@
##           dev_v0.3.0     #131      +/-   ##
==============================================
- Coverage       65.28%   64.69%   -0.59%     
==============================================
  Files              91       91              
  Lines            3272     3308      +36     
  Branches          600      607       +7     
==============================================
+ Hits             2136     2140       +4     
- Misses           1036     1068      +32     
  Partials          100      100              

Flag	Coverage Δ
unittests	64.66% <15.78%> (-0.59%)	:arrow_down:

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

Impacted Files	Coverage Δ
mmrazor/apis/utils.py	27.27% <3.33%> (-28.73%)	:arrow_down:
mmrazor/apis/mmcls/train.py	28.75% <50.00%> (+0.54%)	:arrow_up:
mmrazor/apis/mmdet/train.py	26.92% <50.00%> (+0.60%)	:arrow_up:
mmrazor/apis/mmseg/train.py	35.08% <50.00%> (+0.54%)	:arrow_up:
mmrazor/apis/__init__.py	100.00% <100.00%> (ø)

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