Implement Generative Adversarial Imitation Learning (GAIL)

[nuance1979]

• I have marked all applicable categories:
  • exception-raising fix
  • algorithm implementation fix
  • documentation modification
  • new feature
• I have reformatted the code using make format (required)
• I have checked the code using make commit-checks (required)
• If applicable, I have mentioned the relevant/related issue(s)
• If applicable, I have listed every items in this Pull Request below

Implement GAIL based on PPO and provide example script and sample (i.e., most likely not the best) results with Mujoco tasks. (#531, #173)

[codecov-commenter]

Codecov Report

Merging #550 (77ae3c9) into master (d976a5a) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #550      +/-   ##
==========================================
- Coverage   93.82%   93.81%   -0.01%     
==========================================
  Files          63       64       +1     
  Lines        4322     4368      +46     
==========================================
+ Hits         4055     4098      +43     
- Misses        267      270       +3     

Flag	Coverage Δ
unittests	93.81% <100.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
tianshou/policy/__init__.py	100.00% <100.00%> (ø)
tianshou/policy/imitation/gail.py	100.00% <100.00%> (ø)
tianshou/policy/modelfree/trpo.py	88.52% <0.00%> (-4.92%)	⬇️

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