add register_itertimer_hook function

[MeowZheng]

Fixes #837

[codecov]

Codecov Report

Merging #838 (17a48c2) into master (f75a88c) will decrease coverage by 0.04%.
The diff coverage is 27.27%.

@@            Coverage Diff             @@
##           master     #838      +/-   ##
==========================================
- Coverage   62.93%   62.89%   -0.05%     
==========================================
  Files         145      145              
  Lines        8677     8686       +9     
  Branches     1569     1571       +2     
==========================================
+ Hits         5461     5463       +2     
- Misses       2951     2958       +7     
  Partials      265      265              

Flag	Coverage Δ
unittests	62.89% <27.27%> (-0.05%)	⬇️

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Impacted Files	Coverage Δ
mmcv/runner/base_runner.py	66.51% <27.27%> (-1.86%)	⬇️

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[xiaosai666]

File "/root/***/env1/mmcv/mmcv/runner/base_runner.py", line 423, in register_timer_hook
hook = mmcv.buid_from_cfg(timer_config_, HOOKS)
There's a small bug in line 423

[MeowZheng]

we have fixed it in pr #840 , and will merge it in master.