ci: replace accelerate with diffusers bnb tests

Replace the accelerate job (which had 26/27 tests silently skipping) with
diffusers, a much higher-value integration target for bnb.

Diffusers has two layers of bnb test coverage:
- tests/quantization/bnb/ — standalone 4-bit/8-bit quantization tests
- tests/models/transformers/ — 9 model classes (Flux, HunyuanVideo, Wan,
  etc.) that inherit BitsAndBytesTesterMixin, testing quantization on
  real diffusion model architectures

All are selected via `pytest -m bitsandbytes tests/` which matches the
@is_bitsandbytes decorator that applies pytest.mark.bitsandbytes.

Runner: L40S (bandb-aws-g6e-4xlarge-plus) matching diffusers' own CI
runner (aws-g6e-xlarge-plus). L40S provides 48GB VRAM needed for larger
diffusion models like Flux and HunyuanVideo.

Also sets CUBLAS_WORKSPACE_CONFIG=:16:8 for determinism, matching the
diffusers CI convention.

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

Author: Titus-von-Koeller

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

@@ -154,10 +154,10 @@ jobs:
           path: reports/
           retention-days: 7
 
-  test-accelerate:
-    name: Accelerate bnb tests
+  test-diffusers:
+    name: Diffusers bnb tests
     if: github.repository == 'bitsandbytes-foundation/bitsandbytes'
-    runs-on: bandb-aws-g5-4xlarge-plus-use1-public-80  # A10
+    runs-on: bandb-aws-g6e-4xlarge-plus-use1-public-80  # L40S (matches diffusers CI)
     steps:
       - name: Show GPU information
         run: nvidia-smi
@@ -174,38 +174,40 @@ jobs:
           pip install torch==${TORCH_VERSION} --index-url ${PYPI_INDEX}
           pip install "bitsandbytes[test] @ ${BNB_WHEEL_URL}"
 
-      - name: Install accelerate and clone matching tag
+      - name: Install diffusers and clone matching tag
         run: |
-          pip install accelerate transformers
-          ACCELERATE_VERSION=$(pip show accelerate | awk '/^Version:/ {print $2}')
-          echo "Installed accelerate v${ACCELERATE_VERSION}"
-          git clone --depth=1 --branch "v${ACCELERATE_VERSION}" \
-            https://github.com/huggingface/accelerate.git /tmp/accelerate
+          pip install "diffusers[test]" transformers accelerate peft
+          DIFFUSERS_VERSION=$(pip show diffusers | awk '/^Version:/ {print $2}')
+          echo "Installed diffusers v${DIFFUSERS_VERSION}"
+          git clone --depth=1 --branch "v${DIFFUSERS_VERSION}" \
+            https://github.com/huggingface/diffusers.git /tmp/diffusers
 
       - name: Show environment
         run: |
           pip list
           python -m torch.utils.collect_env
 
-      - name: Run accelerate bnb tests
-        working-directory: /tmp/accelerate
+      - name: Run diffusers bnb tests
+        working-directory: /tmp/diffusers
         env:
           RUN_SLOW: "1"
+          CUBLAS_WORKSPACE_CONFIG: ":16:8"
         shell: bash -o pipefail {0}
         run: |
           mkdir -p ${GITHUB_WORKSPACE}/reports
-          python -m pytest tests/test_quantization.py \
-            -s -v -rs \
-            -k "not multi_device" \
-            --junitxml=${GITHUB_WORKSPACE}/reports/accelerate.xml \
+          python -m pytest \
+            -m bitsandbytes \
+            tests/ \
+            -v \
+            --junitxml=${GITHUB_WORKSPACE}/reports/diffusers.xml \
             -o junit_logging=all \
-            2>&1 | tee ${GITHUB_WORKSPACE}/reports/accelerate.log
+            2>&1 | tee ${GITHUB_WORKSPACE}/reports/diffusers.log
 
       - name: Upload JUnit XML and log
         if: always()
         uses: actions/upload-artifact@v4
         with:
-          name: reports-accelerate
+          name: reports-diffusers
           path: reports/
           retention-days: 7
 
@@ -331,7 +333,7 @@ jobs:
 
   report:
     name: Consolidated report
-    needs: [test-transformers, test-transformers-multigpu, test-accelerate, test-peft, test-peft-multigpu]
+    needs: [test-transformers, test-transformers-multigpu, test-diffusers, test-peft, test-peft-multigpu]
     if: always() && github.repository == 'bitsandbytes-foundation/bitsandbytes'
     runs-on: ubuntu-22.04
     steps: