set_output_data_ptr api (#223)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/223

People really shouldnt be using get_output and mutating the structure, this provides a way to set the output data ptr in a more controlled manner

Reviewed By: iseeyuan

Differential Revision: D49029435

fbshipit-source-id: 44f527d99a0d2c50bbe5a022757adcbd4f7ae20f