[Static Runtime] Move implementation details from impl.h into internal.h (#69274)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69274
`impl.h` is the main header file that defines the interface of Static Runtime to its clients.
However, it is currently filled with implementation details that should not be leaked to our clients. 1) this can unnecessarily leak our internals to our clients which can make it hard to change them later 2) cause unnecessary merge conflicts when multiple people are touching this enormous impl.cpp file.
To alleviate the situation, this change moves the implementation details from impl.h into a new file, internal.h, that's internally kept without leaking the details to our clients.
This change will be followed by another change to rename `impl.h` into `runtime.h` or anything better since `impl.h` is currently not about implementation but SR's interface.
Note that this change is NOT complete since the remaining declarations in impl.h still contain a lot of implementation details. Therefore, we should keep working on minimizing the interface to prevent our API from being bloated unnecessarily. Also we need to work on modularizing our implementations into separate pieces organized by separate files in the near future.
Test Plan: Existing unittests
Reviewed By: donaldong
Differential Revision: D32780415
fbshipit-source-id: 119b7aedbf563b195641c5674572a9348732145f
