[dper3] Export Caffe2 operator LearningRate to PyTorch
Summary: Exports the operator to PyTorch, to be made into a low-level module.
Test Plan:
```
buck test //caffe2/caffe2/python/operator_test:torch_integration_test -- test_learning_rate
```
Reviewed By: yf225
Differential Revision: D23545582
fbshipit-source-id: 6b6d9aa6a47b2802ccef0f87c1263c6cc2d2fdf6
