deduplicated code in elementwise_op_broadcast_test.py (#15865)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15865
factored out code used in tests for operators Add, Mul and Sub
into two new methods: a first one to generate the test vectors, a second
one to run the actual tests given a caffe2 and python operator.
Reviewed By: houseroad
Differential Revision: D13526955
fbshipit-source-id: 8970ba5a1305ca19a54a14b51816d4a19f19d678
