Fixed learning_rate in trainer

[liyongsea]

Variable _learning_rate is wrongly named in trainer.py. I fixed that and I added a test to check this part of the code.

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[liyongsea]

I signed it!

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[codecov-io]

Current coverage is 89.58%

Merging #109 into master will increase coverage by +0.20% as of ad8e499

@@            master    #109   diff @@
======================================
  Files           26      26       
  Stmts          989     989       
  Branches       160     160       
  Methods          0       0       
======================================
+ Hit            884     886     +2
+ Partial         50      49     -1
+ Missed          55      54     -1

Review entire Coverage Diff as of ad8e499

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[terrytangyuan]

Thanks!

[ilblackdragon]

Thanks for fix and especially for the test! :+1: