Codegen: Generate seperate headers per operator (#68247)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68247
This splits `Functions.h`, `Operators.h`, `NativeFunctions.h` and
`NativeMetaFunctions.h` into seperate headers per operator base name.
With `at::sum` as an example, we can include:
```cpp
<ATen/core/sum.h> // Like Functions.h
<ATen/core/sum_ops.h> // Like Operators.h
<ATen/core/sum_native.h> // Like NativeFunctions.h
<ATen/core/sum_meta.h> // Like NativeMetaFunctions.h
```
The umbrella headers are still being generated, but all they do is
include from the `ATen/ops' folder.
Further, `TensorBody.h` now only includes the operators that have
method variants. Which means files that only include `Tensor.h` don't
need to be rebuilt when you modify function-only operators. Currently
there are about 680 operators that don't have method variants, so this
is potentially a significant win for incremental builds.
Test Plan: Imported from OSS
Reviewed By: mrshenli
Differential Revision: D32596272
Pulled By: albanD
fbshipit-source-id: 447671b2b6adc1364f66ed9717c896dae25fa272
