change `drop_on_export` warning category (#29610)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/29610
`DeprecationWarning` is intended for developers (and so is ignored in
certain circumstances). `FutureWarning` is the user-facing deprecation
warning. This fixes fbcode failures.
Test Plan: Imported from OSS
Differential Revision: D18446393
Pulled By: suo
fbshipit-source-id: ded11a007f0a62132a9839b733157a97cf9006e9
