[Feature] Support download inception state from url

[LeoXing1996]

[codecov[bot]]

Codecov Report

Merging #233 (18b5c81) into master (c355edb) will increase coverage by 0.30%. The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #233      +/-   ##
==========================================
+ Coverage   76.71%   77.01%   +0.30%     
==========================================
  Files         130      130              
  Lines        9421     9427       +6     
  Branches     1899     1901       +2     
==========================================
+ Hits         7227     7260      +33     
+ Misses       1728     1703      -25     
+ Partials      466      464       -2     

Flag	Coverage Δ
unittests	77.01% <50.00%> (+0.30%)	:arrow_up:

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

Impacted Files	Coverage Δ
mmgen/core/evaluation/metrics.py	79.08% <50.00%> (+3.56%)	:arrow_up:
mmgen/models/architectures/fid_inception.py	94.44% <0.00%> (+2.38%)	:arrow_up:
mmgen/utils/io_utils.py	58.49% <0.00%> (+5.66%)	:arrow_up:

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[plyfager]

Later we may complete other inception pkls.