introduce a new interface to add op [core changes] (#21147)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21147
This diff introduces a new interface to add PT/C2 operators to the benchmark suite.
The following steps are needed to add a new operator:
1. Specify the input shapes, args to an operator in configs
2. Create a PT/C2 benchmark class which includes ```init``` (create tensors), ```forward``` (specify the operator to be tested.), and ```backward```(gradient of an op.) methods
3. call generate_pt_test/generate_c2_test to create test cases based on configs
Reviewed By: zheng-xq
Differential Revision: D15250380
fbshipit-source-id: 1025a7cf60d2427baa0f3f716455946d3d3e6a27
