Support GPUs in RaySampler

[krzentner]

This has been frequently requested. We've avoided adding support largely because it's difficult to test, and because for many environments sampling on cpu is more efficient anyways, but there are enough bug reports about this it's probably better to just support it.

[codecov]

Codecov Report

Merging #2323 (3af5eeb) into master (c56513f) will decrease coverage by 0.01%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master    #2323      +/-   ##
==========================================
- Coverage   91.28%   91.26%   -0.02%     
==========================================
  Files         199      199              
  Lines       10981    10984       +3     
  Branches     1601     1603       +2     
==========================================
+ Hits        10024    10025       +1     
  Misses        695      695              
- Partials      262      264       +2     

Impacted Files	Coverage Δ
src/garage/sampler/sampler.py	72.72% <ø> (+8.44%)	⬆️
src/garage/sampler/ray_sampler.py	99.21% <88.88%> (+1.69%)	⬆️
...rage/tf/optimizers/conjugate_gradient_optimizer.py	81.90% <0.00%> (-1.81%)	⬇️
src/garage/sampler/multiprocessing_sampler.py	90.12% <0.00%> (-1.24%)	⬇️

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