Make TF Optional

[krzentner]

This should also fix the CI, since it sets more precise version requirements for tensorflow and tensorflow-probability.

[codecov[bot]]

Codecov Report

Merging #2304 (2396d34) into master (7311018) will increase coverage by 0.71%. The diff coverage is 56.25%.

@@            Coverage Diff             @@
##           master    #2304      +/-   ##
==========================================
+ Coverage   90.51%   91.23%   +0.71%     
==========================================
  Files         199      199              
  Lines       11761    10981     -780     
  Branches     1583     1389     -194     
==========================================
- Hits        10646    10018     -628     
+ Misses        837      699     -138     
+ Partials      278      264      -14     

Impacted Files	Coverage Δ
src/garage/torch/algos/sac.py	93.68% <53.33%> (-5.12%)	:arrow_down:
src/garage/sampler/worker_factory.py	92.59% <100.00%> (ø)
src/garage/torch/optimizers/optimizer_wrapper.py	93.75% <0.00%> (-6.25%)	:arrow_down:
src/garage/torch/optimizers/differentiable_sgd.py	86.66% <0.00%> (-3.34%)	:arrow_down:
src/garage/torch/algos/trpo.py	91.66% <0.00%> (-3.08%)	:arrow_down:
...rage/tf/optimizers/conjugate_gradient_optimizer.py	81.90% <0.00%> (-1.81%)	:arrow_down:
src/garage/sampler/multiprocessing_sampler.py	90.12% <0.00%> (-1.24%)	:arrow_down:
src/garage/torch/algos/maml.py	98.18% <0.00%> (-0.55%)	:arrow_down:
src/garage/torch/algos/dqn.py	93.07% <0.00%> (-0.48%)	:arrow_down:
... and 10 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 878a775...2396d34. Read the comment docs.

[krzentner]

This should fix the CI, so I'm force pushing it.