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

[humu789]

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

Please describe the motivation of this PR and the goal you want to achieve through this PR.

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)

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

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%)	⬇️

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%)	⬇️
mmrazor/apis/mmcls/train.py	28.75% <50.00%> (+0.54%)	⬆️
mmrazor/apis/mmdet/train.py	26.92% <50.00%> (+0.60%)	⬆️
mmrazor/apis/mmseg/train.py	35.08% <50.00%> (+0.54%)	⬆️
mmrazor/apis/__init__.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f5ee768...5eea6d3. Read the comment docs.