separate PT and C2 to reduce build time (#28731)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28731
as title
Test Plan:
```
Before:
buck run mode/opt caffe2/benchmarks/operator_benchmark:benchmark_all_test -- --operator sigmoid
Invalidating internal cached state: Buck configuration options changed between invocations. This may cause slower builds.
Changed value project.buck_out='buck-out/opt' (was 'buck-out/dev')
... and 69 more. See logs for all changes
Parsing buck files: finished in 7.2 sec
Creating action graph: finished in 10.0 sec
Building: finished in 06:38.4 min (100%) 29890/29890 jobs, 29890 updated
Total time: 06:55.7 min
# ----------------------------------------
# PyTorch/Caffe2 Operator Micro-benchmarks
# ----------------------------------------
# Tag : short
# Benchmarking PyTorch: sigmoid
With this diff
buck run mode/opt caffe2/benchmarks/operator_benchmark:benchmark_all_test -- --operator sigmoid
Parsing buck files: finished in 6.4 sec
Creating action graph: finished in 9.8 sec
Building: finished in 06:35.9 min (100%) 29892/29892 jobs, 29892 updated
Total time: 06:52.1 min
Reviewed By: hl475
Differential Revision: D18152071
fbshipit-source-id: 80c29570581bbd2f0e78e2df32734c17a2b036ee
