Revert D31945507: .github: Switch 8xlarge to 4xlarge instance_type
Test Plan: revert-hammer
Differential Revision:
D31945507 (https://github.com/pytorch/pytorch/commit/1541bb823ac7398a9e85e5347e06f9924e2942a1)
Original commit changeset: fb8587de7f31
fbshipit-source-id: 3760f5610f0c9cd5298a35490c549e56f7396aaf
