Pin Pillow to v6 as PILLOW_VERSION is removed in v7
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/31777
Test Plan: Imported from OSS
Differential Revision: D19264247
Pulled By: mrshenli
fbshipit-source-id: 52b0a3629e3a96ef2f9d3e289b9f7bb6a2745786
