Update on "[8/n][ET-VK] Support staging any bitw8 image, take 2"

"bitw8" = bit width 8, which is equivalent to 8-bit. We use "bitw8" as the name since shader compilation disallows names starting with a digit.
Changes follow from #4485 to support texture2d and support uint8, respectively.

Differential Revision: [D64076249](https://our.internmc.facebook.com/intern/diff/D64076249/)

[ghstack-poisoned]

Author: jorgep31415