Add torch.no_grad() guard on export (#2373)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2373

For some models where the grad is enabled (like LLava model), we need to disable grad before exporting. This PR is to add this guard when using export script. It would not affect the models that grad is not enabled.

Reviewed By: cccclai

Differential Revision: D54812718

fbshipit-source-id: 639efa5a77839d8d5de52cfa6cb125c883ba54dc