Refine default hooks and custom hooks priority rank.

[mzr1996]

Motivation

Refine default hooks and custom hooks priority rank, and make it be convenient to users.

Modification

Use string to indicate the priority of default hooks, and add more priority ranks.

BC-breaking (Optional)

This PR changes the priority of default hooks of runner, if users already use a priority in their custom hook config, the execution order may change.

Use cases (Optional)

Users can insert a custom hook after optimizer hook by config

custom_hooks = [dict(type='CustomHook', priority='HIGH')]

Checklist

1. Pre-commit or other linting tools are used to fix the potential lint issues.
2. The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
4. The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #1120 (d8a796e) into master (004c006) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1120      +/-   ##
==========================================
+ Coverage   67.62%   67.65%   +0.02%     
==========================================
  Files         159      159              
  Lines       10283    10297      +14     
  Branches     1857     1858       +1     
==========================================
+ Hits         6954     6966      +12     
- Misses       2964     2965       +1     
- Partials      365      366       +1     

Flag	Coverage Δ
unittests	67.65% <100.00%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
mmcv/runner/base_runner.py	77.47% <100.00%> (ø)
mmcv/runner/priority.py	71.42% <100.00%> (+3.00%)	⬆️
mmcv/utils/misc.py	93.57% <0.00%> (-0.92%)	⬇️
mmcv/cnn/utils/weight_init.py	91.34% <0.00%> (-0.04%)	⬇️
mmcv/utils/__init__.py	88.23% <0.00%> (ø)

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[ZwwWayne]

LGTM, see if others have any comments.