add temp allocator for the module (#4009)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4009

Previously the module doesn't set up temp allocator and it was a null ptr. Set up the temp allocator in this diff so I can use it for the sharded model

Reviewed By: mergennachin

Differential Revision: D58736671

fbshipit-source-id: 2e2593e51458da1beb554535a9f769ef10076782