Cortex-M backend: add cortex-m7 to the trunk e2e CI matrix (pytorch#19730)

### Summary
Extends `_test_cortex_m_e2e.yml` with a `targets` input that joins the
existing `model` matrix to give a target × model cross product, so trunk
can exercise the same suite against multiple Cortex-M variants. The
trunk job opts in to `["cortex-m55", "cortex-m7"]`; the nightly job (and
any future caller that doesn't pass `targets`) falls back to the
M55-only default and keeps its current shape.

Cortex-M7 is the first non-MVE variant to enter CI. It exercises the
DSP-class CMSIS-NN kernel paths (selected via `__ARM_FEATURE_DSP`),
covering the build plumbing that threads `-mcpu=cortex-m7` through to
both the runner and the core libraries.

`pull.yml`'s `test-mcu-cortex-m-backend` is intentionally left at
M55-only — per-PR coverage is deferred until the trunk M7 leg
demonstrates stability over a few cycles.

### Test plan
CI

Authored with Claude.

Author: rascani

.ci/scripts/test_cortex_m_e2e.sh

@@ -14,13 +14,14 @@
 set -eu
 
 MODEL=$1
+TARGET=${2:-cortex-m55}
 script_dir=$(realpath "$(dirname "${BASH_SOURCE[0]}")")
 et_root_dir=$(realpath "${script_dir}/../..")
 
-# Quantization is the default for the cortex-m55 target; run.sh's
+# Quantization is the default for cortex-m targets; run.sh's
 # arg parser only recognizes --no_quantize, so we omit any explicit flag.
 export ARM_FVP_INSTALL_I_AGREE_TO_THE_CONTAINED_EULA=True
 bash "${et_root_dir}/examples/arm/run.sh" \
     --model_name="${MODEL}" \
-    --target=cortex-m55 \
+    --target="${TARGET}" \
     --bundleio

.github/workflows/_test_cortex_m_e2e.yml

@@ -11,6 +11,11 @@ on:
         description: 'JSON array of model names to run on the Corstone-300 FVP, e.g. ["mv2", "mv3"]'
         required: true
         type: string
+      targets:
+        description: 'JSON array of cortex-m target CPUs to build the runner for, e.g. ["cortex-m55", "cortex-m7"]'
+        required: false
+        type: string
+        default: '["cortex-m55"]'
       timeout:
         description: 'Per-matrix-entry timeout in minutes'
         required: false
@@ -23,9 +28,10 @@ jobs:
     strategy:
       matrix:
         model: ${{ fromJSON(inputs.models) }}
+        target: ${{ fromJSON(inputs.targets) }}
       fail-fast: false
     with:
-      job-name: ${{ matrix.model }}
+      job-name: ${{ matrix.model }}-${{ matrix.target }}
       runner: linux.2xlarge.memory
       docker-image: ci-image:executorch-ubuntu-22.04-arm-sdk
       submodules: 'recursive'
@@ -44,4 +50,4 @@ jobs:
         source examples/arm/arm-scratch/setup_path.sh
 
         # Export and run model on FVP (run.sh internally builds the test runner).
-        bash .ci/scripts/test_cortex_m_e2e.sh ${{ matrix.model }}
+        bash .ci/scripts/test_cortex_m_e2e.sh "${{ matrix.model }}" "${{ matrix.target }}"

.github/workflows/trunk.yml

@@ -1075,3 +1075,4 @@ jobs:
     uses: ./.github/workflows/_test_cortex_m_e2e.yml
     with:
       models: '["mv2", "mv3"]'
+      targets: '["cortex-m55", "cortex-m7"]'