Add support for int8 textures and buffers (#3892)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/3892

## Context

As title. This changeset adds support for Tensors that have the dtype `api::kChar` or `api::kQInt8` for both buffer and texture storage.
ghstack-source-id: 229394337
exported-using-ghexport

Reviewed By: jorgep31415

Differential Revision: D58263388

fbshipit-source-id: f73574e663d846b5a333f476ab7b4b842768c4e2