ptez_loader (#3897)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3897

This diff creates a data loader, called PtezDataLoader, to load ptez file during the runtime.
For this diff, PtezDataLoader is only focusing on loading from disk every time we invoke Load function.

Reviewed By: tarun292

Differential Revision: D58133351

fbshipit-source-id: a2a8bdd55c36eed5faafb12613d5ad061b19f1a1