[Doc] Move public TensorFlow runtime APIs to new file. (#23032)
* [Doc] Move public TensorFlow runtime APIs to new file.
Move public APIs from `CompilerRuntime.swift` to new file `Execution.swift`.
This clearly separates user-exposed public functions from compiler internals.
This separation also benefits `jazzy` API doc generation: `jazzy` can simply
skip doc generation for the entire `CompilerRuntime.swift` file. Otherwise,
per-declaration `/// :nodoc:` annotations would be necessary for skipping
documentation.
* Add clarification comment to `CompilerRuntime.swift`.
Address feedback from @mhong.
* Improve clarification comment.
