[Fix] Register optimizer constructor with mmseg

[linfangjian01]

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Motivation

Fix to register optimizer constructor directly into mmcv.

Modification

1. Add mmseg/core/builder.py to register OPTIMIZER_BUILDERS of mmcv.
2. Change the build_optimizer import from mmcv to mmseg in mmseg/apis/train.py.
3. Change the OPTIMIZER_BUILDERS import from mmcv to mmseg in mmseg/core/utils/layer_decay_optimizer_constructor.py and mmseg/core/layer_decay_optimizer_constructor.py.

[codecov]

Codecov Report

Merging #1456 (bb3548a) into master (23ae1eb) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1456      +/-   ##
==========================================
+ Coverage   90.24%   90.30%   +0.06%     
==========================================
  Files         139      140       +1     
  Lines        8310     8335      +25     
  Branches     1397     1400       +3     
==========================================
+ Hits         7499     7527      +28     
+ Misses        572      570       -2     
+ Partials      239      238       -1     

Flag	Coverage Δ
unittests	90.30% <100.00%> (+0.06%)	⬆️

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Impacted Files	Coverage Δ
mmseg/apis/train.py	19.51% <100.00%> (ø)
mmseg/core/__init__.py	100.00% <100.00%> (ø)
mmseg/core/builder.py	100.00% <100.00%> (ø)
mmseg/core/layer_decay_optimizer_constructor.py	88.63% <100.00%> (+0.26%)	⬆️
...eg/core/utils/layer_decay_optimizer_constructor.py	94.87% <100.00%> (+0.06%)	⬆️
mmseg/models/losses/cross_entropy_loss.py	86.90% <0.00%> (+4.18%)	⬆️

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[MeowZheng]

Please revise the tag [Fix] -> [WIP], as WIP (working in process) means this pr is not ready to review