[Arch] Remove torch_dispatch frontend and move figure to docs/ dir (#1372)

Addresses leftover comment from earlier PRs: #1254 , #1265 to remove `torch_dispatch` frontend. In addition, moves the main arch diagram into `docs/` directory for consistency.

Author: sjain-stanford

README.md

@@ -22,7 +22,7 @@ Multiple Vendors use MLIR as the middle layer, mapping from platform frameworks
 
 We have few paths to lower down to the Torch MLIR Dialect.
 
-![Torch Lowering Architectures](Torch-MLIR.png)
+![Torch Lowering Architectures](docs/Torch-MLIR.png)
 
  - TorchScript
     This is the most tested path down to Torch MLIR Dialect, and the PyTorch ecosystem is converging on using TorchScript IR as a lingua franca.