[Fix] Fix SyncBN build in PyTorch 1.9

[xvjiarui]

Motivation

Fix #1137

Modification

Check whether attribute _specify_ddp_gpu_num exists.

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?

No

[xvjiarui]

We may need CI for PyTorch 1.9. @zhouzaida

[zhouzaida]

yet, we are resolving the compatibility in PyTorch1.9

[codecov]

Codecov Report

Merging #1138 (6843309) into master (1b15f02) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 6843309 differs from pull request most recent head d9af6b0. Consider uploading reports for the commit d9af6b0 to get more accurate results

@@           Coverage Diff           @@
##           master    #1138   +/-   ##
=======================================
  Coverage   67.96%   67.96%           
=======================================
  Files         159      159           
  Lines       10419    10416    -3     
  Branches     1897     1896    -1     
=======================================
- Hits         7081     7079    -2     
  Misses       2969     2969           
+ Partials      369      368    -1     

Flag	Coverage Δ
unittests	67.96% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
mmcv/utils/parrots_wrapper.py	60.71% <ø> (-0.31%)	⬇️
mmcv/cnn/bricks/norm.py	100.00% <100.00%> (ø)

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[ZwwWayne]

Seems this PR does not totally fix #1137 here :

mmcv/mmcv/utils/parrots_wrapper.py

Lines 83 to 87 in eb08835

	class SyncBatchNorm(SyncBatchNorm_):
	def _specify_ddp_gpu_num(self, gpu_size):
	if TORCH_VERSION != 'parrots':
	super()._specify_ddp_gpu_num(gpu_size)

[zhouzaida]

We should check why pytorch1.9 removes the function, please refer to Remove _specify_ddp_gpu_num method

[xvjiarui]

We should check why pytorch1.9 removes the function, please refer to Remove _specify_ddp_gpu_num method

They remove it because it's not useful any more. The attribute _ddp_gpu_size is also removed.