Refactor stablehlo API and put them in official location. (#5493)
Changes include:
* make end point in torch_xla/init.py for exposed APIs torch_xla.save_as_stablehlo and torch_xla.save_torch_model_as_stablehlo.
* All tf related integration to its own file.
* Remove args as argument (because it will spear inside of ExportedProgram) but allow user to override it (which we use for now.
