Make plotting plot the learner in BC

[krzentner]

Before, using BC with an expert source policy caused the expert to be shown in the plotter, which was very confusing. This change fixes that, by making the expert the algorithm's .exploration_policy and the learner its .policy

[codecov[bot]]

Codecov Report

Merging #2127 into master will decrease coverage by 0.01%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2127      +/-   ##
==========================================
- Coverage   93.16%   93.14%   -0.02%     
==========================================
  Files         194      194              
  Lines       10523    10525       +2     
  Branches     1323     1323              
==========================================
  Hits         9804     9804              
- Misses        473      476       +3     
+ Partials      246      245       -1     

Impacted Files	Coverage Δ
src/garage/torch/algos/bc.py	92.10% <100.00%> (+0.21%)	:arrow_up:
src/garage/sampler/multiprocessing_sampler.py	89.61% <0.00%> (-1.30%)	:arrow_down:

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[krzentner]

@Mergifyio rebase

[mergify[bot]]

Command rebase: success

Branch has been successfully rebased

[krzentner]

@mergifyio rebase

[mergify[bot]]

Command rebase: success

Branch has been successfully rebased