Disable MNIST test in test_xla() (#34261)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/34261
Test Plan: Imported from OSS
Differential Revision: D20260350
Pulled By: mrshenli
fbshipit-source-id: b92a6b79e59bdfdf8e68b5dd73f87ea1dfd0daed
