mujoco benchmark -- ddpg, td3, sac

[ChenDRAG]

Releasing Tianshou's SOTA benchmark of 9 out of 13 environments from the MuJoCo Gym task suite.
Last commit of #274 finally! See #274 for details.

[ChenDRAG]

@yingchengyang

[codecov-io]

Codecov Report

Merging #305 (1a1fed5) into master (389bdb7) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #305      +/-   ##
==========================================
- Coverage   94.04%   94.00%   -0.04%     
==========================================
  Files          51       51              
  Lines        3256     3237      -19     
==========================================
- Hits         3062     3043      -19     
  Misses        194      194              

Flag	Coverage Δ
unittests	94.00% <100.00%> (-0.04%)	⬇️

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

Impacted Files	Coverage Δ
tianshou/policy/modelfree/ddpg.py	98.78% <100.00%> (+0.07%)	⬆️
tianshou/policy/modelfree/dqn.py	98.78% <100.00%> (ø)
tianshou/policy/modelfree/sac.py	85.71% <100.00%> (-1.79%)	⬇️
tianshou/policy/modelfree/td3.py	100.00% <100.00%> (ø)
tianshou/trainer/offline.py	100.00% <100.00%> (ø)
tianshou/trainer/offpolicy.py	100.00% <100.00%> (ø)
tianshou/trainer/onpolicy.py	97.01% <100.00%> (ø)

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