Add gpu pickleable module to torch

[ziyiwu9494]

A potential fix for #2079, by creating an garage.torch.Module wrapper class which implements pickling with moving to cpu on pickle and back to original device on unpickle if the device exists.

[codecov[bot]]

Codecov Report

Merging #2265 (3654cf0) into master (90b6090) will decrease coverage by 0.02%. The diff coverage is 82.60%.

@@            Coverage Diff             @@
##           master    #2265      +/-   ##
==========================================
- Coverage   91.20%   91.17%   -0.03%     
==========================================
  Files         199      199              
  Lines       11627    10996     -631     
  Branches     1557     1392     -165     
==========================================
- Hits        10604    10026     -578     
+ Misses        754      703      -51     
+ Partials      269      267       -2     

Impacted Files	Coverage Δ
src/garage/torch/_functions.py	90.84% <80.95%> (-3.49%)	:arrow_down:
src/garage/torch/policies/policy.py	100.00% <100.00%> (ø)
src/garage/torch/optimizers/optimizer_wrapper.py	93.75% <0.00%> (-6.25%)	:arrow_down:
src/garage/sampler/env_update.py	82.92% <0.00%> (-3.62%)	:arrow_down:
src/garage/torch/optimizers/differentiable_sgd.py	86.66% <0.00%> (-3.34%)	:arrow_down:
src/garage/torch/policies/stochastic_policy.py	88.63% <0.00%> (-3.17%)	:arrow_down:
src/garage/torch/algos/trpo.py	91.66% <0.00%> (-3.08%)	:arrow_down:
src/garage/torch/algos/dqn.py	93.07% <0.00%> (-2.09%)	:arrow_down:
src/garage/np/_functions.py	77.39% <0.00%> (-1.74%)	:arrow_down:
... and 8 more

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