[Refactor] refactor EvalHook

[dreamerlin]

OverView

This PR refactors EvalHook

1. Add by_epoch param to determine perform evaluation by epoch or by iteration.
2. Add automatically assign a compare key when users do not set one.
3. Save checkpoint by creating a symlink as well as record best info in runner hook meta
4. Refactor the code structure

Migration

Configs files need to be changed

1. Rename EpochEvalHook to EvalHook, DistEpochEvalHook to DistEvalHook.
2. Rename key_indicator to save_best.

For example, in slowfast_kinetics_pretrained_r50_4x16x1_20e_ava_rgb.py cfg file

from

evaluation = dict(interval=1, key_indicator='[email protected]')

to

evaluation = dict(interval=1, save_best='[email protected]')

Misc.

This Part may further be merged into MMCV or applied to other MM codebases.

[codecov]

Codecov Report

Merging #395 (cbfbf82) into master (b747208) will increase coverage by 0.07%.
The diff coverage is 85.14%.

@@            Coverage Diff             @@
##           master     #395      +/-   ##
==========================================
+ Coverage   84.76%   84.84%   +0.07%     
==========================================
  Files         121      121              
  Lines        8579     8604      +25     
  Branches     1416     1427      +11     
==========================================
+ Hits         7272     7300      +28     
+ Misses        953      947       -6     
- Partials      354      357       +3     

Flag	Coverage Δ
unittests	84.83% <85.14%> (+0.07%)	⬆️

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

Impacted Files	Coverage Δ
mmaction/apis/train.py	13.63% <50.00%> (ø)
mmaction/core/evaluation/eval_hooks.py	88.11% <84.44%> (+4.64%)	⬆️
mmaction/core/evaluation/__init__.py	100.00% <100.00%> (ø)
mmaction/datasets/activitynet_dataset.py	92.79% <100.00%> (+0.06%)	⬆️
mmaction/datasets/base.py	60.71% <100.00%> (ø)
mmaction/datasets/hvu_dataset.py	96.10% <100.00%> (+0.05%)	⬆️
mmaction/datasets/ssn_dataset.py	92.72% <100.00%> (+0.02%)	⬆️

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[innerlee]

Please help with user migration.

• Add a stub for the old classes DistEpochEvalHook, EpochEvalHook, and give proper error messages (ideally, also give links to this pr) to guide the user to update configs/codes.
• Give detailed migration guide on the top post in this thread. Like I commented at Refactor ava hook #567 (comment)

[dreamerlin]

Please help with user migration.

• Add a stub for the old classes DistEpochEvalHook, EpochEvalHook, and give proper error messages (ideally, also give links to this pr) to guide the user to update configs/codes.
• Give detailed migration guide on the top post in this thread. Like I commented at #567 (comment)

Done