[ExecuTorch][WebGPU] Tests for the adamw WebGPU ops

Pull Request resolved: #20936

Export-delegation + fp64 golden tests for the adamw (`adamw_step`) ops.

Key changes:
- `test/ops/test_adamw.py` — fp64 in-place param/m/v goldens (incl. non-zero weight-decay) + delegation checks — fp64 library-reference goldens + delegation checks (≥2 cases/op).

Co-authored-with: Claude Code.
ghstack-source-id: 405026082
@exported-using-ghexport

Differential Revision: [D111755127](https://our.internmc.facebook.com/intern/diff/D111755127/)

Author: JCNTH

backends/webgpu/test/ops/test_adamw.py

@@ -0,0 +1,142 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the BSD-style license found in the
+# LICENSE file in the root directory of this source tree.
+
+"""AdamW optimizer step (`et_vk.adamw_step`) export + fp64 golden.
+
+`adamw_step(param, m, v, grad, lr, beta1, beta2, eps, weight_decay, bc1, bc2)`
+updates the fp32 latent in place: decoupled weight decay, then the bias-corrected
+Adam moment update. `bc1`/`bc2` (= 1 - beta^t) are host-precomputed so the kernel
+carries no step counter. The op mutates and returns `param`/`m`/`v` (aliased), so
+export wraps it in `auto_functionalized`, which VulkanPartitioner tags by name (the
+same mutating-op path as `update_cache`). Golden is the fp64 reference, computed
+independently so a lossy fp32 op impl cannot fake-pass.
+"""
+
+from __future__ import annotations
+
+import unittest
+from dataclasses import dataclass
+
+import torch
+
+from executorch.backends.vulkan.partitioner.vulkan_partitioner import VulkanPartitioner
+from executorch.exir import to_edge_transform_and_lower
+
+# torch.optim.AdamW defaults; bc1/bc2 are 1 - beta^step (host-precomputed).
+LR = 1e-3
+BETA1 = 0.9
+BETA2 = 0.999
+EPS = 1e-8
+
+
+@dataclass(frozen=True)
+class AdamwConfig:
+    name: str
+    numel: int
+    weight_decay: float = 0.01
+    step: int = 1
+
+
+CONFIGS = [
+    AdamwConfig("small", 64),
+    AdamwConfig("no_wd", 256, weight_decay=0.0),  # wd=0 -> the plain Adam path
+    AdamwConfig("later_step", 1000, step=10),  # bias correction well past t=1
+]
+
+
+def _inputs(cfg: AdamwConfig):
+    """Deterministic fp32 param/m/v/grad + the host bias corrections."""
+    g = torch.Generator().manual_seed(0)
+    param = torch.randn(cfg.numel, generator=g, dtype=torch.float32)
+    m = torch.randn(cfg.numel, generator=g, dtype=torch.float32) * 0.1
+    v = torch.rand(cfg.numel, generator=g, dtype=torch.float32) * 0.01
+    grad = torch.randn(cfg.numel, generator=g, dtype=torch.float32)
+    bc1 = 1.0 - BETA1**cfg.step
+    bc2 = 1.0 - BETA2**cfg.step
+    return param, m, v, grad, bc1, bc2
+
+
+def _fp64_golden(param, m, v, grad, wd, bc1, bc2):
+    """fp64 truth for one AdamW step; mirrors adamw_step.wgsl exactly."""
+    p = param.double()
+    g = grad.double()
+    p = p - LR * wd * p
+    m64 = BETA1 * m.double() + (1.0 - BETA1) * g
+    v64 = BETA2 * v.double() + (1.0 - BETA2) * g * g
+    mhat = m64 / bc1
+    vhat = v64 / bc2
+    p = p - LR * mhat / (torch.sqrt(vhat) + EPS)
+    return p.float(), m64.float(), v64.float()
+
+
+class _AdamwModule(torch.nn.Module):
+    def __init__(self, wd: float, bc1: float, bc2: float) -> None:
+        super().__init__()
+        self.wd = wd
+        self.bc1 = bc1
+        self.bc2 = bc2
+
+    def forward(self, param, m, v, grad):
+        return torch.ops.et_vk.adamw_step(
+            param, m, v, grad, LR, BETA1, BETA2, EPS, self.wd, self.bc1, self.bc2
+        )
+
+
+def _export(cfg: AdamwConfig):
+    param, m, v, grad, bc1, bc2 = _inputs(cfg)
+    ep = torch.export.export(
+        _AdamwModule(cfg.weight_decay, bc1, bc2), (param, m, v, grad)
+    )
+    return to_edge_transform_and_lower(
+        ep, partitioner=[VulkanPartitioner()]
+    ).to_executorch()
+
+
+def _delegates(et) -> bool:
+    return any(
+        d.id == "VulkanBackend"
+        for plan in et.executorch_program.execution_plan
+        for d in plan.delegates
+    )
+
+
+class TestAdamwStep(unittest.TestCase):
+    def test_export_delegates(self) -> None:
+        for cfg in CONFIGS:
+            with self.subTest(config=cfg.name):
+                et = _export(cfg)
+                self.assertTrue(
+                    _delegates(et), f"no VulkanBackend delegate in {cfg.name}"
+                )
+
+    def test_op_matches_fp64_golden(self) -> None:
+        for cfg in CONFIGS:
+            with self.subTest(config=cfg.name):
+                param, m, v, grad, bc1, bc2 = _inputs(cfg)
+                g_param, g_m, g_v = _fp64_golden(
+                    param, m, v, grad, cfg.weight_decay, bc1, bc2
+                )
+                # Op mutates in place; clone so the golden saw the originals.
+                out_param, out_m, out_v = torch.ops.et_vk.adamw_step(
+                    param.clone(),
+                    m.clone(),
+                    v.clone(),
+                    grad,
+                    LR,
+                    BETA1,
+                    BETA2,
+                    EPS,
+                    cfg.weight_decay,
+                    bc1,
+                    bc2,
+                )
+                torch.testing.assert_close(out_param, g_param, atol=5e-4, rtol=1e-3)
+                torch.testing.assert_close(out_m, g_m, atol=5e-4, rtol=1e-3)
+                torch.testing.assert_close(out_v, g_v, atol=5e-4, rtol=1e-3)
+
+
+if __name__ == "__main__":
+    unittest.main()