[NNC] Add C++ codegen backend to NNC (#62869)
Summary:
Adds a C++ codegen backend to NNC to generate C++ for CPU instead of generating LLVM IR.
Tensors are represented as blobs of float. Vector operations are devectorized/unrolled.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/62869
Test Plan:
https://github.com/pytorch/pytorch/tree/mvz-nnc-aot-prototype makes it able to AOT compile the whole MobileNetV3 model into binary code through LLVM codegen in NNC.
I forked that branch to https://github.com/cheng-chang/pytorch/tree/cc-aot-cpp, merged this PR into it, and modified `fancy_compile` to compile MobileNetV3 into C++ through
```
import torch
m = torch.jit.load('mobnet.pt')
m.eval()
f = torch.jit.freeze(m)
torch._C._fancy_compile(f.graph, [1, 3, 224, 224])
```
The generated C++ file `mobnet.cc` can be found at https://gist.github.com/cheng-chang/e2830cc6920b39204ebf368035b2bcec.
I manually compiled the generated C++ through `g++ -o mobnet -std=c++14 -L./build/lib -ltorch_cpu -ltorch mobnet.cc`, and it succeeded.
Reviewed By: ZolotukhinM
Differential Revision: D30149482
Pulled By: cheng-chang
fbshipit-source-id: e77b189f0353e37cd309423a48a513e668d07675
