Change tf trainer import

[codecov[bot]]

Codecov Report

Merging #2271 (d1d6f1f) into master (90b6090) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           master    #2271    +/-   ##
========================================
  Coverage   91.23%   91.23%            
========================================
  Files         199      199            
  Lines       11627    10968   -659     
  Branches     1557     1387   -170     
========================================
- Hits        10608    10007   -601     
+ Misses        753      698    -55     
+ Partials      266      263     -3     

Impacted Files	Coverage Δ
src/garage/trainer.py	94.81% <100.00%> (+2.11%)	:arrow_up:
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/torch/_functions.py	92.42% <0.00%> (-1.92%)	:arrow_down:
...rage/tf/optimizers/conjugate_gradient_optimizer.py	81.90% <0.00%> (-1.81%)	:arrow_down:
src/garage/np/_functions.py	77.39% <0.00%> (-1.74%)	:arrow_down:
... and 8 more

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