[Feature] LAMB optimizer

[0x4f5da2]

Motivation

The 'ResNet Strikes Back' requires LAMB optimizer

Modification

Add LAMB optimizer

[codecov]

Codecov Report

Merging #591 (858260d) into master (58ab028) will decrease coverage by 0.83%.
The diff coverage is 0.00%.

❗ Current head 858260d differs from pull request most recent head 3182a22. Consider uploading reports for the commit 3182a22 to get more accurate results

@@            Coverage Diff             @@
##           master     #591      +/-   ##
==========================================
- Coverage   80.01%   79.17%   -0.84%     
==========================================
  Files         109      111       +2     
  Lines        6419     6487      +68     
  Branches     1102     1117      +15     
==========================================
  Hits         5136     5136              
- Misses       1146     1214      +68     
  Partials      137      137              

Flag	Coverage Δ
unittests	79.17% <0.00%> (-0.84%)	⬇️

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Impacted Files	Coverage Δ
mmcls/core/optimizers/__init__.py	0.00% <0.00%> (ø)
mmcls/core/optimizers/lamb.py	0.00% <0.00%> (ø)

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[mzr1996]

LGTM