Flag to disable Variable
Summary:
using `buck build mode/opt mode/no-gpu //experimental/ngimel/benchmark_framework_overheads:cpp_benchmark`
```
devvm497.prn3.facebook.com:/data/users/bwasti/fbsource/fbcode $ ./cpp_benchmark --niter 10000
creating inputs, number of dimensions 1
starting op
benchmarking 10000 iterations
using cpp frontend
elapsed time per iteration 0.90638 us
```
```
devvm497.prn3.facebook.com:/data/users/bwasti/fbsource/fbcode $ ./cpp_benchmark --niter 10000 --disable_variable_dispatch
creating inputs, number of dimensions 1
starting op
benchmarking 10000 iterations
using cpp frontend
elapsed time per iteration 0.775436 us
```
Test Plan: let all tests run
Reviewed By: smessmer
Differential Revision: D18654276
fbshipit-source-id: 362812b2c87ec428448b2ac65baac45f492fdce4
