Add OSS CI for the CPU backend test suite flow (#24252)

Summary:

Run the backend test suite's `cpu_fp32` flow in OSS CI the way XNNPACK, Vulkan and WebGPU run theirs, now that `EXECUTORCH_BUILD_CPU` builds `CpuBackend` into the pybindings.

Reviewed By: shoumikhin

Differential Revision: D127905371

Pull Request resolved: #24252

Author: JakeStevens

.ci/scripts/test_backend.sh

@@ -53,6 +53,10 @@ if [[ "$FLOW" == *qnn* ]]; then
     EXTRA_BUILD_ARGS+=" -DEXECUTORCH_BUILD_QNN=ON -DQNN_SDK_ROOT=$QNN_SDK_ROOT -DEXECUTORCH_BUILD_EXTENSION_TENSOR=ON"
 fi
 
+if [[ "$FLOW" == *cpu* ]]; then
+    EXTRA_BUILD_ARGS+=" -DEXECUTORCH_BUILD_CPU=ON -DEXECUTORCH_BUILD_XNNPACK=ON -DEXECUTORCH_BUILD_KERNELS_OPTIMIZED=ON"
+fi
+
 if [[ "$FLOW" == *vulkan* ]]; then
     # CPU runners can have Mesa ICDs installed without a usable hardware GPU.
     source .ci/scripts/setup-vulkan-linux-deps.sh "swiftshader"

.github/workflows/_test_backend.yml

@@ -4,7 +4,7 @@ on:
   workflow_call:
     inputs:
       backend:
-        description: 'Backend to test (xnnpack, coreml, vulkan, qnn, openvino)'
+        description: 'Backend to test (xnnpack, coreml, vulkan, qnn, openvino, cpu)'
         required: true
         type: string
       flows:

.github/workflows/test-backend-cpu.yml

@@ -0,0 +1,31 @@
+name: Test CPU Backend
+
+on:
+  schedule:
+    - cron: 0 2 * * *
+  push:
+    branches:
+      - main
+      - release/*
+    tags:
+      - ciflow/nightly/*
+  pull_request:
+  workflow_dispatch:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref_name }}-${{ github.ref_type == 'branch' && github.sha }}-${{ github.event_name == 'workflow_dispatch' }}-${{ github.event_name == 'schedule' }}
+  cancel-in-progress: true
+
+jobs:
+  test-cpu:
+    uses: ./.github/workflows/_test_backend.yml
+    permissions:
+      id-token: write
+      contents: read
+    with:
+      backend: cpu
+      flows: '["cpu_fp32"]'
+      ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
+      timeout: 120
+      run-linux: true
+      docker-image: ci-image:executorch-ubuntu-22.04-gcc11