[Enhancement] Refine the docstring of ResNet

[Junjun2016]

Motivation

Some args of ResNet in the docstring are missing.
Some default values of args are also missing

Modification

Docstring of ResNet

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos?
No

[codecov]

Codecov Report

Merging #723 (813fd66) into master (4d34581) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #723   +/-   ##
=======================================
  Coverage   85.28%   85.28%           
=======================================
  Files         107      107           
  Lines        5817     5817           
  Branches      952      952           
=======================================
  Hits         4961     4961           
  Misses        673      673           
  Partials      183      183           

Flag	Coverage Δ
unittests	85.26% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmseg/models/backbones/resnet.py	99.28% <ø> (ø)

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