Revert "add amp support for custom backend (#96188)"
This reverts commit cf12edee02a44009c4f06e36efa97d9a7372ab35.
Reverted https://github.com/pytorch/pytorch/pull/96188 on behalf of https://github.com/kit1980 due to Broke some linalg tests : https://github.com/pytorch/pytorch/actions/runs/4420037607/jobs/7750708339
