Add dtype arg to the script for exporting HuggingFace models (#5716)

Summary:
As titled. This will unblock delegating to XNNPACK w/ float16 and bfloat16, which provides a comparable perf data points against egear, torch.compile, and AOTI, etc.

Pull Request resolved: https://github.com/pytorch/executorch/pull/5716

Reviewed By: kirklandsign

Differential Revision: D63499648

Pulled By: guangy10

fbshipit-source-id: 5a06454f8af664e6d5f469dcf63869ca7c57a6ba