Add learning rate scheduler to BasePolicy

[alexnikulkov]

• 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

#586

The BasePolicy class gets a new optional argument which can be either a single LR scheduler or multiple LR schedulers wrapped as MultipleLRSchedulers(scheduler_1, scheduler_2, ...)

[codecov-commenter]

Codecov Report

Merging #598 (86b181d) into master (6fc6857) will increase coverage by 0.03%.
The diff coverage is 77.77%.

@@            Coverage Diff             @@
##           master     #598      +/-   ##
==========================================
+ Coverage   93.51%   93.54%   +0.03%     
==========================================
  Files          66       67       +1     
  Lines        4426     4433       +7     
==========================================
+ Hits         4139     4147       +8     
+ Misses        287      286       -1     

Flag	Coverage Δ
unittests	93.54% <77.77%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
tianshou/policy/imitation/base.py	100.00% <ø> (ø)
tianshou/policy/imitation/bcq.py	100.00% <ø> (ø)
tianshou/policy/imitation/cql.py	100.00% <ø> (ø)
tianshou/policy/imitation/discrete_bcq.py	98.24% <ø> (ø)
tianshou/policy/imitation/discrete_cql.py	100.00% <ø> (ø)
tianshou/policy/imitation/discrete_crr.py	93.44% <ø> (ø)
tianshou/policy/imitation/gail.py	100.00% <ø> (ø)
tianshou/policy/modelbased/icm.py	97.95% <ø> (ø)
tianshou/policy/modelbased/psrl.py	97.40% <ø> (ø)
tianshou/policy/modelfree/a2c.py	98.46% <ø> (+1.44%)	⬆️
... and 17 more

📣 Codecov can now indicate which changes are the most critical in Pull Requests. Learn more