Fix errors due to SB change

[shwang]

VecNormalize._normalize_observation was renamed

[codecov]

Codecov Report

Merging #143 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #143   +/-   ##
=======================================
  Coverage   85.63%   85.63%           
=======================================
  Files          59       59           
  Lines        4052     4052           
=======================================
  Hits         3470     3470           
  Misses        582      582

Impacted Files	Coverage Δ
src/imitation/algorithms/adversarial.py	89.62% <100%> (ø)	⬆️
src/imitation/policies/serialize.py	98.55% <100%> (ø)	⬆️
tests/test_rollout.py	100% <0%> (ø)	⬆️
tests/test_tabular.py	100% <0%> (ø)	⬆️
tests/test_scripts.py	100% <0%> (ø)	⬆️
src/imitation/scripts/expert_demos.py	87.34% <0%> (ø)	⬆️
tests/test_buffer.py	100% <0%> (ø)	⬆️
src/imitation/scripts/config/parallel.py	63.46% <0%> (ø)	⬆️
src/imitation/util/buffer.py	97.36% <0%> (ø)	⬆️

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