[Enhancement] Add to_ntuple

[Junjun2016]

Add to_ntuple

[codecov]

Codecov Report

Merging #1125 (39719fc) into master (004c006) will increase coverage by 0.02%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master    #1125      +/-   ##
==========================================
+ Coverage   67.62%   67.64%   +0.02%     
==========================================
  Files         159      159              
  Lines       10283    10296      +13     
  Branches     1857     1858       +1     
==========================================
+ Hits         6954     6965      +11     
- Misses       2964     2965       +1     
- Partials      365      366       +1     

Flag	Coverage Δ
unittests	67.64% <85.71%> (+0.02%)	⬆️

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Impacted Files	Coverage Δ
mmcv/utils/misc.py	93.57% <84.61%> (-0.92%)	⬇️
mmcv/utils/__init__.py	88.23% <100.00%> (ø)

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