Update Torch-MLIR Architecture Diagram (#1254)

Add MHLO path
Add custom accelarator dialects
Rename Torch Dialect back to original Torch-MLIR Dialect
(Surrounding text still refers to Torch-MLIR dialect)
Check in source for Excalidraw(https://excalidraw.com/)
so anyone can use / update it using the open source version

Author: powderluv