[caffe2] Speed up compilation of aten-op.cc (#44440)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44440
`aten-op.cc` takes a long time to compile due to the large generated constructor. For each case, the `std::function` constructor and the initialization functions are inlined, producing a huge amount of intermediate code that takes a long time to optimize, given that many compiler optimization passes are superlinear in the function size.
This diff moves each case to a separate function, so that each one is cheap to optimize, and the constructor is just a large jump table, which is easy to optimize.
Reviewed By: dzhulgakov
Differential Revision: D23593741
fbshipit-source-id: 1ce7a31cda10d9b0c9d799716ea312a291dc0d36
