ci: use diffusers Docker image for diffusers job, add axolotl job

Use diffusers/diffusers-pytorch-cuda container for the diffusers test
job — matches their own CI setup and has all system deps (libGL, etc.)
pre-installed. Only override bnb with our continuous-release wheel.
Previous bare-metal approach failed with libGL.so.1 missing during
pytest collection (opencv-python needs it, and bare-metal runners
don't have it).

If the runner doesn't have Docker/nvidia-container-toolkit, the job
will fail fast with a clear error and we'll fall back to installing
opencv-python-headless instead.

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

Author: Titus-von-Koeller

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

@@ -157,25 +157,21 @@ jobs:
     name: Diffusers bnb tests
     if: github.repository == 'bitsandbytes-foundation/bitsandbytes'
     runs-on: bandb-aws-g6e-4xlarge-plus-use1-public-80  # L40S (matches diffusers CI)
+    container:
+      image: diffusers/diffusers-pytorch-cuda
+      options: --gpus all --shm-size "16gb" --ipc host
     steps:
       - name: Show GPU information
         run: nvidia-smi
 
       - uses: actions/checkout@v4
 
-      - name: Setup Python
-        uses: actions/setup-python@v5
-        with:
-          python-version: ${{ env.PYTHON_VERSION }}
-
-      - name: Install torch + bnb (from continuous-release)
+      - name: Install bnb (from continuous-release) over the image's version
         run: |
-          pip install torch==${TORCH_VERSION} --index-url ${PYPI_INDEX}
-          pip install "bitsandbytes[test] @ ${BNB_WHEEL_URL}"
+          pip install --force-reinstall --no-deps ${BNB_WHEEL_URL}
 
-      - name: Install diffusers and clone matching tag
+      - name: Clone diffusers matching installed version
         run: |
-          pip install "diffusers[test]" transformers accelerate peft
           DIFFUSERS_VERSION=$(pip show diffusers | awk '/^Version:/ {print $2}')
           echo "Installed diffusers v${DIFFUSERS_VERSION}"
           git clone --depth=1 --branch "v${DIFFUSERS_VERSION}" \