ci : add workflow for testing CPU variants [no ci]

This is a work in progress to try out different CPU variants using the
CPU reference backend with repacking enabled. The workflow will be
manually triggered and allows specifying the operation and variant
to test.

Author: danbev

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

@@ -0,0 +1,59 @@
+name: Test CPU Variants
+
+on:
+  workflow_dispatch:
+    inputs:
+      operation:
+        description: 'Operation to test (e.g., MUL_MAT or full spec)'
+        required: false
+        default: 'MUL_MAT'
+        type: string
+      variant:
+        description: 'CPU variant to test (leave empty to list available variants)'
+        required: false
+        default: ''
+        type: string
+
+jobs:
+  test-cpu-variant:
+    runs-on: [self-hosted, Linux, X64, CPU, AMX]
+    steps:
+      - name: Clone
+        uses: actions/checkout@v4
+
+      - name: Build with CPU reference backend
+        run: |
+          cmake -B build -S . \
+            -DGGML_CPU_REF_BACKEND=ON \
+            -DGGML_CPU_ALL_VARIANTS=ON \
+            -DGGML_CPU_REPACK=ON \
+            -DGGML_NATIVE=OFF \
+            -DGGML_BACKEND_DL=ON \
+            -DGGML_BLAS=OFF \
+            -DCMAKE_BUILD_TYPE=Release
+          
+          cmake --build build -j8
+
+      - name: List available CPU variants
+        run: |
+          echo "Available CPU variants:"
+          ./build/bin/test-backend-ops cpu-variants --list
+
+      - name: Test CPU variant
+        if: ${{ inputs.variant != '' }}
+        run: |
+          echo "Testing variant: ${{ inputs.variant }}"
+          echo "Operation: ${{ inputs.operation }}"
+          ./build/bin/test-backend-ops cpu-variants \
+            --variant ${{ inputs.variant }} \
+            -o "${{ inputs.operation }}"
+
+      - name: Instructions
+        if: ${{ inputs.variant == '' }}
+        run: |
+          echo "=========================================="
+          echo "No variant specified - only listed available variants above"
+          echo "To test a specific variant, re-run this workflow with:"
+          echo "  - variant: one of the variants listed above"
+          echo "  - operation: your operation string (default: MUL_MAT)"
+          echo "=========================================="