[Fix] _init_dist_slurm can fail if scontrol gives a warning

[TimDarcet]

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Motivation

subprocess.getoutput also returns stderr. If scontrol issues a warning, the variable addr will include this warning.
Using subprocess.check_output fixes this.
Reference:
[1]:https://docs.python.org/2/library/subprocess.html#subprocess.check_output

Modification

Use subprocess.check_output instead of subprocess.getoutput in mmcv.runner._init_dist_slurm

Checklist

Before PR:

• I have read and followed the workflow indicated in the CONTRIBUTING.md to create this PR.
• Pre-commit or linting tools indicated in CONTRIBUTING.md are used to fix the potential lint issues.
• Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
• New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
• The documentation has been modified accordingly, including docstring or example tutorials.

After PR:

• If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
• CLA has been signed and all committers have signed the CLA in this PR.

[CLAassistant]

All committers have signed the CLA.

[imabackstabber]

Thanks for your contribution.I add a ref link,which explain subprocess.check_output.

[imabackstabber]

LGTM.

[imabackstabber]

maybe need to fix some lint @TimDarcet

[imabackstabber]

maybe need to fix some lint @TimDarcet

I change it anyway.

[ZwwWayne]

unit tests failed

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