ci: add multi-GPU transformers job mirroring legacy peft pattern

The legacy peft nightly-bnb.yml (deleted in peft#2858) ran the full
transformers bnb suite on both its single- and multi-GPU jobs, so the
multi-GPU-marked tests in transformers' test_4bit.py / test_mixed_int8.py
actually executed on the 2-GPU runner. This commit restores that coverage.

New job: test-transformers-multigpu
- Runner: bandb-aws-g6-12xlarge-plus (same multi-GPU runner as peft)
- CUDA_VISIBLE_DEVICES=0,1 (2 of the 4 L4s)
- Filter: `-k "MultiGpu or multi_gpu"` — runs ONLY the multi-GPU-marked
  tests, avoiding duplication with the single-GPU transformers job.

Report job's `needs:` updated.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Titus-von-Koeller

.github/workflows/tests-integration-nightly.yml

@@ -98,6 +98,62 @@ jobs:
           path: reports/
           retention-days: 7
 
+  test-transformers-multigpu:
+    name: Transformers bnb tests (multi GPU)
+    if: github.repository == 'bitsandbytes-foundation/bitsandbytes'
+    runs-on: bandb-aws-g6-12xlarge-plus-use1-public-80  # 4× L4 (2 used)
+    steps:
+      - name: Show GPU information
+        run: nvidia-smi
+
+      - uses: actions/checkout@v4
+
+      - name: Setup Python
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ env.PYTHON_VERSION }}
+
+      - name: Install torch + bnb (from continuous-release)
+        run: |
+          pip install torch==${TORCH_VERSION} --index-url ${PYPI_INDEX}
+          pip install "bitsandbytes[test] @ ${BNB_WHEEL_URL}"
+
+      - name: Install transformers and clone matching tag
+        run: |
+          pip install transformers accelerate
+          TRANSFORMERS_VERSION=$(pip show transformers | awk '/^Version:/ {print $2}')
+          echo "Installed transformers v${TRANSFORMERS_VERSION}"
+          git clone --depth=1 --branch "v${TRANSFORMERS_VERSION}" \
+            https://github.com/huggingface/transformers.git /tmp/transformers
+
+      - name: Show environment
+        run: |
+          pip list
+          python -m torch.utils.collect_env
+
+      - name: Run transformers bnb tests (multi-GPU only)
+        working-directory: /tmp/transformers
+        env:
+          RUN_SLOW: "1"
+          CUDA_VISIBLE_DEVICES: "0,1"
+        shell: bash -o pipefail {0}
+        run: |
+          mkdir -p ${GITHUB_WORKSPACE}/reports
+          python -m pytest tests/quantization/bnb/ \
+            -v \
+            -k "MultiGpu or multi_gpu" \
+            --junitxml=${GITHUB_WORKSPACE}/reports/transformers-multigpu.xml \
+            -o junit_logging=all \
+            2>&1 | tee ${GITHUB_WORKSPACE}/reports/transformers-multigpu.log
+
+      - name: Upload JUnit XML and log
+        if: always()
+        uses: actions/upload-artifact@v4
+        with:
+          name: reports-transformers-multigpu
+          path: reports/
+          retention-days: 7
+
   test-accelerate:
     name: Accelerate bnb tests
     if: github.repository == 'bitsandbytes-foundation/bitsandbytes'
@@ -280,7 +336,7 @@ jobs:
 
   report:
     name: Consolidated report
-    needs: [test-transformers, test-accelerate, test-peft, test-peft-multigpu]
+    needs: [test-transformers, test-transformers-multigpu, test-accelerate, test-peft, test-peft-multigpu]
     if: always() && github.repository == 'bitsandbytes-foundation/bitsandbytes'
     runs-on: ubuntu-22.04
     steps: