Change the input of `mvit_v2_s` on the FX test (#83242)
Addresses some [breakages](https://github.com/pytorch/pytorch/runs/7782559841?check_suite_focus=true) from #82560
Context: The tests are breaking because a new architecture was added in TorchVision (see https://github.com/pytorch/vision/pull/6373) that requires a different input size. This PR addresses it by using the right size for the `mvit_v2_s` architecture.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/83242
Approved by: https://github.com/ezyang
