Fix benchmark_caffe2
Summary: benchmakr_caffe2 is broken, due to some refactoring which change from eager test generation to register only.
Test Plan:
`buck run caffe2/benchmarks/operator_benchmark/c2:add_test`
```
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short
# Benchmarking Caffe2: add
WARNING: Logging before InitGoogleLogging() is written to STDERR
W1021 08:07:06.350742 390665 init.h:137] Caffe2 GlobalInit should be run before any other API calls.
# Name: add_M8_N16_K32_dtypeint
# Input: M: 8, N: 16, K: 32, dtype: int
Forward Execution Time (us) : 652.748
# Benchmarking Caffe2: add
# Name: add_M16_N16_K64_dtypefloat
# Input: M: 16, N: 16, K: 64, dtype: float
Forward Execution Time (us) : 63.570
# Benchmarking Caffe2: add
# Name: add_M64_N64_K128_dtypeint
# Input: M: 64, N: 64, K: 128, dtype: in
```
Reviewed By: qizzzh
Differential Revision: D24448374
fbshipit-source-id: 850fd375d194c20c385ea4433aea13066c7476e6
