[dper3] Create dper LearningRate low-level module
Summary: As title; this will unblock migration of several modules that need learning rate functionality.
Test Plan:
```
buck test //dper3/dper3/modules/low_level_modules/tests:learning_rate_test
```
WIP: need to add more learning rate tests for the different policies
Reviewed By: yf225
Differential Revision: D23584071
fbshipit-source-id: f6656531b1caba38c3e3a7d6e16d9591563391e2
