Add imitation baselines for offline RL

[nuance1979]

• I have marked all applicable categories:
  • exception-raising fix
  • algorithm implementation fix
  • documentation modification
  • new feature
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add imitation baselines for offline RL; make the choice of env/task and D4RL dataset explicit; on expert datasets, IL easily outperforms; after reading the D4RL paper, I'll rerun the exps on medium data

[codecov-commenter]

Codecov Report

Merging #566 (8c365c6) into master (74f430e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #566   +/-   ##
=======================================
  Coverage   93.78%   93.78%           
=======================================
  Files          64       64           
  Lines        4376     4376           
=======================================
  Hits         4104     4104           
  Misses        272      272           

Flag	Coverage Δ
unittests	93.78% <100.00%> (ø)

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Impacted Files	Coverage Δ
tianshou/env/worker/ray.py	85.36% <100.00%> (ø)

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[Trinkle23897]

Is it ready and could you please check the d4rl scripts can successfully run?

[nuance1979]

Is it ready and could you please check the d4rl scripts can successfully run?

Yes, it's ready. I have run the scripts successfully and put the results in README.md .