refactor save_data as non member function (#42045)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42045
This PR changes the save_data() member functions of torch::jit::mobile::Module which was introduced in #41403 to be the non member function torch::jit::mobile::_save_parameters() (taking a mobile Module as its first argument).
In addition, this PR:
* adds a getter function _ivalue() for the mobile::Module object
* renames torch::jit::mobile::_load_mobile_data() to torch::jit::mobile_load_parameters()
* refactors the import.h header file into import.h and import_data.h
Test Plan: Imported from OSS
Reviewed By: kwanmacher, iseeyuan
Differential Revision: D22766781
Pulled By: ann-ss
fbshipit-source-id: 5cabae31927187753a958feede5e9a28d71d9e92
