[JIT][to_backend] Move code that is not related to the user-facing API out of jit/backends/backend.h #38431
Description
Background
JIT recently added a feature that allows a Module to be lowered for execution on a non-JIT specialized backend, such as Glow. The API for declaring specialized backends is in torch/csrc/jit/backends/backend.h. However, there is code in there that is not templated (e.g. getPreprocessSchema) and does not need to be in that header. Its continued presence can be confusing for end-users.
The Task
Move this code that does not need to be in backend.h into a detail namespace in a separate header or into a separate cpp file.
How to Test
python test/test_jit.py TestBackends
cc @suo