fix parrots op bug

[luopeichao]

Fix sync_bn bug in parrots. And when use parrots, open custom op exporting.

[codecov]

Codecov Report

Merging #1289 (cb76458) into master (ac0d839) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head cb76458 differs from pull request most recent head 1ec3b7a. Consider uploading reports for the commit 1ec3b7a to get more accurate results

@@           Coverage Diff           @@
##           master    #1289   +/-   ##
=======================================
  Coverage   68.42%   68.43%           
=======================================
  Files         160      160           
  Lines       10610    10612    +2     
  Branches     1938     1938           
=======================================
+ Hits         7260     7262    +2     
  Misses       2975     2975           
  Partials      375      375           

Flag	Coverage Δ
unittests	68.43% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
mmcv/image/photometric.py	99.27% <ø> (ø)
mmcv/onnx/info.py	72.22% <100.00%> (+3.47%)	⬆️

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[zhouzaida]

LGTM