[Feature] Add layer decay optimizer constructor and learning rate decay optimizer constructor

[jin-s13]

Motivation

1. Add layer decay optimizer constructor
2. Add learning rate decay optimizer constructor
   These two optimizers will be used in BEiT, MAE, ViT, ConvNeXt backbones.

The codes are from mmsegmentation.

Related PRs: open-mmlab/mmsegmentation@f16bb06

Modification

1. rename mmpose/core/optimizer --> mmpose/core/optimizers
2. Add mmpose/core/optimizers/layer_decay_optimizer_constructor.py
3. move tests/test_optimizer.py to tests/test_core/test_optimizer.py

BC-breaking (Optional)

Use cases (Optional)

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit tests to ensure correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• CLA has been signed and all committers have signed the CLA in this PR.

[codecov]

Codecov Report

Merging #1423 (6008dff) into master (fef003a) will increase coverage by 0.06%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master    #1423      +/-   ##
==========================================
+ Coverage   84.35%   84.42%   +0.06%     
==========================================
  Files         232      233       +1     
  Lines       19279    19391     +112     
  Branches     3468     3500      +32     
==========================================
+ Hits        16262    16370     +108     
  Misses       2152     2152              
- Partials      865      869       +4     

Flag	Coverage Δ
unittests	84.35% <96.55%> (+0.07%)	⬆️

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Impacted Files	Coverage Δ
...re/optimizers/layer_decay_optimizer_constructor.py	96.11% <96.11%> (ø)
mmpose/core/__init__.py	100.00% <100.00%> (ø)
mmpose/core/optimizers/__init__.py	100.00% <100.00%> (ø)
mmpose/core/optimizers/builder.py	100.00% <100.00%> (ø)

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