Add PER

[maliesa96]

Closes #1489.

[maliesa96]

Benchmarks should be done in a few days.

[codecov]

Codecov Report

❌ Patch coverage is 88.23529% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.52%. Comparing base (e1c576f) to head (8f3fb8e).
⚠️ Report is 59 commits behind head on master.

Files with missing lines	Patch %	Lines
src/garage/torch/algos/dqn.py	61.53%	4 Missing and 1 partial ⚠️
src/garage/torch/algos/pearl.py	0.00%	2 Missing ⚠️
src/garage/replay_buffer/per_replay_buffer.py	97.61%	0 Missing and 1 partial ⚠️

❌ Your patch check has failed because the patch coverage (88.23%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2159      +/-   ##
==========================================
- Coverage   91.56%   91.52%   -0.04%     
==========================================
  Files         197      198       +1     
  Lines       10927    10979      +52     
  Branches     1387     1393       +6     
==========================================
+ Hits        10005    10049      +44     
- Misses        663      669       +6     
- Partials      259      261       +2     

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[ryanjulian]

How are the benchmark results?

[krzentner]

Nice. This is a pretty clean implementation.

[maliesa96]

Currently rerunning some of the benchmarks (some were interrupted), will report back once they're done.