Advertise USE_PRECOMPILED_HEADERS in CONTRIBUTING.md (#62827)

Summary:
This option was added in https://github.com/pytorch/pytorch/issues/61940 and fits with this section's theme of improving build times.

I've also changed it to a `cmake_dependent_option` instead of `FATAL_ERROR`ing for older CMake versions.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/62827

Reviewed By: astaff

Differential Revision: D30342102

Pulled By: malfet

fbshipit-source-id: 3095b44b7085aee8a884ec95cba9f8998d4442e7

CMakeLists.txt

@@ -241,10 +241,9 @@ option(USE_OBSERVERS "Use observers module." OFF)
 option(USE_OPENCL "Use OpenCL" OFF)
 option(USE_OPENCV "Use OpenCV" OFF)
 option(USE_OPENMP "Use OpenMP for parallel code" ON)
-option(USE_PRECOMPILED_HEADERS "Use pre-compiled headers to accelerate build. Requires cmake >= 3.16." OFF)
-if(USE_PRECOMPILED_HEADERS AND (CMAKE_VERSION VERSION_LESS "3.16"))
-  message(FATAL_ERROR "Precompiled headers require cmake >= 3.16")
-endif()
+cmake_dependent_option(
+    USE_PRECOMPILED_HEADERS "Use pre-compiled headers to accelerate build. Requires cmake >= 3.16." OFF
+    "CMAKE_VERSION VERSION_LESS \"3.16\"" OFF)
 
 option(USE_PROF "Use profiling" OFF)
 option(USE_QNNPACK "Use QNNPACK (quantized 8-bit operators)" ON)

CONTRIBUTING.md

@@ -30,6 +30,7 @@
     - [Use Ninja](#use-ninja)
     - [Use CCache](#use-ccache)
     - [Use a faster linker](#use-a-faster-linker)
+    - [Use pre-compiled headers](#use-pre-compiled-headers)
   - [C++ frontend development tips](#c-frontend-development-tips)
   - [GDB integration](#gdb-integration)
 - [CUDA development tips](#cuda-development-tips)
@@ -850,6 +851,27 @@ The easiest way to use `lld` this is download the
 ln -s /path/to/downloaded/ld.lld /usr/local/bin/ld
 ```
 
+#### Use pre-compiled headers
+
+Sometimes there's no way of getting around rebuilding lots of files, for example
+editing `native_functions.yaml` usually means 1000+ files being rebuilt. If
+you're using CMake newer than 3.16, you can enable pre-compiled headers by
+setting `USE_PRECOMPILED_HEADERS=1` either on first setup, or in the
+`CMakeCache.txt` file.
+
+```sh
+USE_PRECOMPILED_HEADERS=1 python setup.py develop
+```
+
+This adds a build step where the compiler takes `<ATen/ATen.h>` and essentially
+dumps it's internal AST to a file so the compiler can avoid repeating itself for
+every `.cpp` file.
+
+One caveat is that when enabled, this header gets included in every file by default.
+Which may change what code is legal, for example:
+- internal functions can never alias existing names in `<ATen/ATen.h>`
+- names in `<ATen/ATen.h>` will work even if you don't explicitly include it.
+
 ### C++ frontend development tips
 
 We have very extensive tests in the [test/cpp/api](test/cpp/api) folder. The