Correct /MP usage in MSVC (#33120)
Summary:
## Several flags
`/MP[M]`: It is a flag for the compiler `cl`. It leads to object-level multiprocessing. By default, it spawns M processes where M is the number of cores on the PC.
`/maxcpucount:[M]`: It is a flag for the generator `msbuild`. It leads to project-level multiprocessing. By default, it spawns M processes where M is the number of cores on the PC.
`/p:CL_MPCount=[M]`: It is a flag for the generator `msbuild`. It leads the generator to pass `/MP[M]` to the compiler.
`/j[M]`: It is a flag for the generator `ninja`. It leads to object-level multiprocessing. By default, it spawns M processes where M is the number of cores on the PC.
## Reason for the change
1. Object-level multiprocessing is preferred over project-level multiprocessing.
2. ~For ninja, we don't need to set `/MP` otherwise M * M processes will be spawned.~ Actually, it is not correct because in ninja configs, there are only one source file in the command. Therefore, the `/MP` switch should be useless.
3. For msbuild, if it is called through Python configuration scripts, then `/p:CL_MPCount=[M]` will be added, otherwise, we add `/MP` to `CMAKE_CXX_FLAGS`.
4. ~It may be a possible fix for https://github.com/pytorch/pytorch/issues/28271, https://github.com/pytorch/pytorch/issues/27463 and https://github.com/pytorch/pytorch/issues/25393. Because `/MP` is also passed to `nvcc`.~ It is probably not true. Because `/MP` should not be effective given there is only one source file per command.
## Reference
1. https://docs.microsoft.com/en-us/cpp/build/reference/mp-build-with-multiple-processes?view=vs-2019
2. https://github.com/Microsoft/checkedc-clang/wiki/Parallel-builds-of-clang-on-Windows
3. https://blog.kitware.com/cmake-building-with-all-your-cores/
Pull Request resolved: https://github.com/pytorch/pytorch/pull/33120
Differential Revision: D19817227
Pulled By: ezyang
fbshipit-source-id: f8d01f835016971729c7a8d8a0d1cb8a8c2c6a5f
