[c2] Fix int types for learning rate
Summary: Currently GetSingleArgument is overflowing since it's expecting an int instead of an int64 when using a 1cycle (hill policy) annealing schedule
Test Plan:
unittest
buck test caffe2/caffe2/python/operator_test:learning_rate_op_test
Differential Revision: D23938169
fbshipit-source-id: 20d65df800d7a0f1dd9520705af31f63ae716463
