Fix ExecutorBackend output value assignment (#4020)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4020

In the previous version, we re-assign the Evalue pointer instead of set the Evalue pointer content, it's problematic as the pointer will be destructed when the nested executor is destructed.

In this diff we fix it by copying the tensor value

Reviewed By: tarun292

Differential Revision: D58739622

fbshipit-source-id: 7f83f5299d5aa68cad8a095a138d46aed07f7810