Refresh backend README with progress timeline (#20115)

Summary:

Update the WebGPU backend README to reflect the current state of the
backend:

- Add a Progress section listing milestones landed on `main`
  (#18808, #19963, #19964, #19981, #20036) and work in review
  (#20079, #20080), each linking its pull request.
- Update the operator support table to include `rms_norm` and refresh
  the planned/roadmap list toward end-to-end LLM inference.
- Update the directory structure to match the current layout.

Docs-only change; no code or build impact.

Differential Revision: D107742574

Author: JulianCloudNTH

backends/webgpu/README.md

@@ -2,7 +2,26 @@
 
 Run ExecuTorch models on the GPU via [WebGPU](https://www.w3.org/TR/webgpu/). The backend compiles delegated subgraphs into WGSL compute shaders executed natively through [wgpu-native](https://github.com/gfx-rs/wgpu-native) (Metal on macOS, Vulkan on Linux/Windows).
 
-> **Status: Prototype.** The backend supports a single operator today and is under active development. See [TODO.md](TODO.md) for the roadmap.
+> **Status: Prototype.** The backend supports `add` and `rms_norm` today and is under active development. See [Progress](#progress) for shipped milestones.
+
+## Progress
+
+Milestones landed on `main`:
+
+| Date | Milestone | Pull Request |
+|---|---|---|
+| 2026-04 | Made it possible to run ExecuTorch models on the GPU through WebGPU — built the backend from the ground up, including the runtime delegate that builds the GPU graph (buffers, pipelines, bind groups) and runs the model on Metal and Vulkan | [#18808](https://github.com/pytorch/executorch/pull/18808) |
+| 2026-06 | Grew model support beyond element-wise operators — added the root-mean-square normalization operator (`rms_norm`) and named-data weight loading | [#19963](https://github.com/pytorch/executorch/pull/19963) |
+| 2026-06 | Made sure every change is automatically tested — added WebGPU to ExecuTorch's standard backend test suite, running on Linux/x86 in CI | [#19964](https://github.com/pytorch/executorch/pull/19964) |
+| 2026-06 | Removed a class of bugs and manual upkeep — the WGSL shaders are now generated automatically, with a build-time check that fails the build on shader/source drift | [#19981](https://github.com/pytorch/executorch/pull/19981) |
+| 2026-06 | Got the test suite to actually run work on the GPU — added operator-allowlist delegation (unsupported operations fall back to the CPU) and a process-wide GPU device context, so models execute on the GPU during testing | [#20036](https://github.com/pytorch/executorch/pull/20036) |
+
+In review:
+
+| Milestone | Pull Request |
+|---|---|
+| Makes testing match the WebGPU standard exactly — switches the tests to Google's Dawn shader compiler (Tint, the source-of-truth WGSL implementation) running on SwiftShader for headless GPU execution | [#20079](https://github.com/pytorch/executorch/pull/20079) |
+| Strengthens correctness for models that run in several GPU passes — adds dispatch-ordering and scratch-buffer (temporary GPU memory) tests | [#20080](https://github.com/pytorch/executorch/pull/20080) |
 
 ## Architecture
 
@@ -36,8 +55,9 @@ Key design choices:
 | Operator | WGSL Shader | Notes |
 |---|---|---|
 | `aten.add.Tensor` | `binary_add.wgsl` | Element-wise with alpha: `out = in1 + alpha * in2` |
+| `et_vk.rms_norm.default` | `rms_norm.wgsl` | Root-mean-square normalization |
 
-**Planned:** `sub`, `mul`, `relu`, `linear` (matmul), `softmax`, `layer_norm`
+**Planned:** scaled-dot-product attention (KV cache), quantized linear (4-bit weight-only and 8da4w post-training quantization), quantized embedding, RoPE, `mul`, `sigmoid`, and shape ops (`view`, `permute`, `slice`, `select`, `cat`, `squeeze`/`unsqueeze`).
 
 ## Quick Start
 
@@ -83,27 +103,37 @@ This runs Python export tests, exports a .pte, builds the native runtime, and va
 backends/webgpu/
 ├── CMakeLists.txt
 ├── README.md
-├── TODO.md
 ├── runtime/
 │   ├── WebGPUBackend.h/cpp        # BackendInterface (init/execute)
 │   ├── WebGPUGraph.h/cpp          # GPU graph: buffers, pipelines, dispatch
 │   ├── WebGPUDelegateHeader.h/cpp # VH00 header parser
 │   ├── WebGPUDevice.h/cpp         # wgpu-native device abstraction
+│   ├── WebGPUUtils.h              # Workgroup-size helpers
 │   └── ops/
 │       ├── OperatorRegistry.h/cpp # Op dispatch table
-│       └── add/
-│           ├── BinaryOp.cpp       # aten.add.Tensor implementation
-│           ├── binary_add.wgsl    # WGSL shader source
-│           └── binary_add_wgsl.h  # Shader as C++ string constant
+│       ├── add/
+│       │   ├── BinaryOp.cpp       # aten.add.Tensor implementation
+│       │   ├── binary_add.wgsl    # WGSL shader source
+│       │   └── binary_add_wgsl.h  # Shader as C++ string constant
+│       └── rms_norm/
+│           ├── RmsNorm.cpp        # et_vk.rms_norm implementation
+│           ├── rms_norm.wgsl      # WGSL shader source
+│           └── rms_norm_wgsl.h    # Shader as C++ string constant
 ├── scripts/
-│   └── setup-wgpu-native.sh      # Download wgpu-native binaries
+│   ├── setup-wgpu-native.sh      # Download wgpu-native binaries
+│   └── gen_wgsl_headers.py       # Generate the embedded *_wgsl.h shader headers
 └── test/
     ├── conftest.py
+    ├── tester.py                  # Partitioner stages + supported-op list
     ├── test_build_webgpu.sh       # End-to-end build + test
     ├── test_webgpu_native.cpp     # C++ native test runner
-    └── ops/
-        └── add/
-            └── test_add.py        # Python export tests
+    ├── test_wgsl_codegen.py       # Shader codegen check
+    ├── native/                    # C++ operator tests
+    └── ops/                       # Python export tests
+        ├── add/
+        │   └── test_add.py        # add export tests
+        └── rms_norm/
+            └── test_rms_norm.py   # rms_norm export tests
 ```
 
 ## Requirements