[Fix] Add sn_style option for SAGAN and SNGAN.

[LeoXing1996]

In #159, we support Ajbrock's spectral norm, and the default style of spectral norm is changed.
Therefore we support users to select the style of spectral norm and set the default style to torch.

[codecov]

Codecov Report

Merging #199 (9769916) into master (697817f) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #199   +/-   ##
=======================================
  Coverage   75.61%   75.62%           
=======================================
  Files         128      128           
  Lines        8927     8938   +11     
  Branches     1781     1783    +2     
=======================================
+ Hits         6750     6759    +9     
- Misses       1728     1730    +2     
  Partials      449      449           

Flag	Coverage Δ
unittests	75.62% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
...rchitectures/sngan_proj/generator_discriminator.py	88.32% <100.00%> (+0.21%)	⬆️
mmgen/models/architectures/sngan_proj/modules.py	98.30% <100.00%> (+0.91%)	⬆️
...chitectures/stylegan/generator_discriminator_v1.py	88.70% <0.00%> (-1.13%)	⬇️
...chitectures/stylegan/generator_discriminator_v2.py	86.66% <0.00%> (-1.12%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 697817f...9769916. Read the comment docs.