Create a workflow to test PennyLane with NumPy 1.26 (#6275)

**Context:** In #6061, we started using by default NumPy 2 (instead of
NumPy 1) on the CI to test every PR targeting the master branch. Some
jobs (for example, those requiring `tensorflow`) still automatically
downgrade to NumPy 1.26 since not all interfaces are supported at the
current stage.

This PR aims to create another workflow to test some jobs with NumPy
1.26 (those that don't automatically downgrade to NumPy 1 during the
day). Such a workflow should run at night to ensure PennyLane is still
compatible with NumPy 1.26.

**We'll keep this workflow only temporarily** (probably a quarter, but
this still has to be established). After that, we'll get rid of it.

Finally, It has been verified (see
[here](https://github.com/PennyLaneAI/pennylane/actions/runs/10927439038))
that the tests introduced with this PR are green. This has been verified
a second time (see
[here](https://github.com/PennyLaneAI/pennylane/actions/runs/10946347640)
before merging this PR into master).

**Description of the Change:** As above.

**Benefits:** We ensure that PennyLane is still compatible with NumPy
1.26.

**Possible Drawbacks:** 

- This issue was discovered accidentally (and potentially fixed) during
this PR. The `TestSinglePrecision` class in the `test_jax_jit_qnode.py`
file contains tests that change the JAX precision with
`jax.config.update("jax_enable_x64", False)`. When running the jax tests
using multiple workers, we can face race conditions if some workers
change jax configuration, because other workers in other test classes
downgrade to a single precision causing numerous failures. To avoid
this, we add
[--dist=loadscope](https://pytest-xdist.readthedocs.io/en/latest/distribution.html)
as additional pytest arg for jax tests, grouping tests by module for
test functions and by class for test methods.

- The new workflow is it is to a large extent a (very simplified) copy
of `interface-unit-tests.yml`. This is against the principle of avoiding
code duplication whenever possible. Still, this is just a temporary
workflow that lasts for a few months at most (and runs only at night),
and it may be convenient to disentangle it as much as possible from the
usual test interface.

**Related GitHub Issues:** None.

**Related Shortcut Stories:** [sc-73744]

.github/workflows/install_deps/action.yml

@@ -8,6 +8,11 @@ inputs:
     description: The version of Python to use in order to run unit tests
     required: false
     default: '3.10'
+  install_numpy_1:
+    description: Indicate if numpy 1 should be installed or not
+    required: false
+    type: boolean
+    default: false  
   install_jax:
     description: Indicate if JAX should be installed or not
     required: false
@@ -67,6 +72,11 @@ runs:
         pip install -r requirements-ci.txt --upgrade
         pip install -r requirements-dev.txt --upgrade
 
+    - name: Install numpy 1 version
+      shell: bash
+      if: inputs.install_numpy_1 == 'true'
+      run: pip install "numpy==1.26.4"  
+
     - name: Install PyTorch
       shell: bash
       if: inputs.install_pytorch == 'true'

.github/workflows/interface-unit-tests.yml

@@ -270,7 +270,7 @@ jobs:
       install_pennylane_lightning_master: true
       pytest_coverage_flags: ${{ inputs.pytest_coverage_flags }}
       pytest_markers: jax and not qcut and not finite-diff and not param-shift
-      pytest_additional_args: --splits 5 --group ${{ matrix.group }}
+      pytest_additional_args: --dist=loadscope --splits 5 --group ${{ matrix.group }}
       pytest_durations_file_path: '.github/workflows/jax_tests_durations.json'
       pytest_store_durations: ${{ inputs.pytest_store_durations }}
       additional_pip_packages: pytest-split

.github/workflows/numpy_1_tests.yml

@@ -0,0 +1,19 @@
+name: NumPy 1 - Tests
+on:
+
+  schedule:
+    # Runs daily at 6 AM UTC (1 AM Toronto in winter, 2 AM in summer)
+    - cron: '0 6 * * *'  
+
+  # Allows to run this workflow manually from the Actions tab
+  workflow_dispatch:
+
+concurrency:
+  group: numpy-1-unit-tests-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  tests:
+    uses: ./.github/workflows/numpy_1_unit-tests.yml
+    with:
+      branch: ${{ github.ref }}

.github/workflows/numpy_1_unit-tests.yml

@@ -0,0 +1,230 @@
+# TODO: remove this workflow once compatibility with NumPy 1
+# will no longer be tested.
+
+name: NumPy 1 - Unit Tests - Interfaces
+on:
+  workflow_call:
+    inputs:
+      branch:
+        description: The PennyLane branch to checkout and run unit tests for
+        required: true
+        type: string
+      pipeline_mode:
+          description: The pipeline mode can be unit-tests, benchmarks, or reference-benchmarks
+          required: false
+          type: string
+          default: 'unit-tests'
+      disable_new_opmath:
+        description: Whether to disable the new op_math or not when running the tests
+        required: false
+        type: string
+        default: "False"
+
+jobs:
+  setup-ci-load:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Setup Python Versions
+        id: python_versions
+
+        run: |
+            cat >python_versions.json <<-EOF
+            {
+                "default": ["3.10", "3.11", "3.12"],
+                "torch-tests": ["3.10", "3.12"],
+                "jax-tests": ["3.10", "3.12"],
+                "external-libraries-tests": ["3.10"],
+                "data-tests": ["3.10"]
+            }
+            EOF
+        
+            jq . python_versions.json
+            echo "python_versions=$(jq -r tostring python_versions.json)" >> $GITHUB_OUTPUT
+
+      - name: Set NumPy Version
+        id: numpy_version
+        run: echo "NUMPY_VERSION=1.26" >> $GITHUB_ENV     
+
+      - name: Setup Matrix Max Parallel
+        id: max_parallel
+        run: |
+            cat >matrix_max_parallel.json <<-EOF
+            {
+                "default": 1,
+                "core-tests": 5,
+                "jax-tests": 10,
+                "torch-tests": 2
+            }
+            EOF
+
+            jq . matrix_max_parallel.json
+            echo "matrix_max_parallel=$(jq -r tostring matrix_max_parallel.json)" >> $GITHUB_OUTPUT
+
+    outputs:
+      matrix-max-parallel: ${{ steps.max_parallel.outputs.matrix_max_parallel }}
+      python-version: ${{ steps.python_versions.outputs.python_versions }}
+
+  torch-tests:
+    needs:
+      - setup-ci-load
+    strategy:
+      max-parallel: >-
+        ${{
+           fromJSON(needs.setup-ci-load.outputs.matrix-max-parallel).torch-tests
+           || fromJSON(needs.setup-ci-load.outputs.matrix-max-parallel).default
+         }}
+      matrix:
+        python-version: >-
+          ${{
+            fromJSON(needs.setup-ci-load.outputs.python-version).torch-tests
+            || fromJSON(needs.setup-ci-load.outputs.python-version).default
+           }}
+    uses: ./.github/workflows/unit-test.yml
+    with:
+      job_name: torch-tests (${{ matrix.python-version }}, numpy-1.26)
+      branch: ${{ inputs.branch }}
+      coverage_artifact_name: core-interfaces-coverage-torch-${{ matrix.python-version }}-numpy-1.26
+      python_version: ${{ matrix.python-version }}
+      pipeline_mode: ${{ inputs.pipeline_mode }}
+      install_numpy_1: true
+      install_jax: false
+      install_tensorflow: false
+      install_pytorch: true
+      install_pennylane_lightning_master: true
+      pytest_markers: torch and not qcut and not finite-diff and not param-shift
+      requirements_file: ${{ github.event_name == 'schedule' && strategy.job-index == 0 && 'torch.txt' || '' }}
+      disable_new_opmath: ${{ inputs.disable_new_opmath }}
+
+
+  autograd-tests:
+    needs:
+      - setup-ci-load
+    strategy:
+      max-parallel: >-
+        ${{
+           fromJSON(needs.setup-ci-load.outputs.matrix-max-parallel).autograd-tests
+           || fromJSON(needs.setup-ci-load.outputs.matrix-max-parallel).default
+         }}
+      matrix:
+        python-version: >-
+          ${{
+            fromJSON(needs.setup-ci-load.outputs.python-version).autograd-tests
+            || fromJSON(needs.setup-ci-load.outputs.python-version).default
+           }}
+    uses: ./.github/workflows/unit-test.yml
+    with:
+      job_name: autograd-tests (${{ matrix.python-version }}, numpy-1.26)
+      branch: ${{ inputs.branch }}
+      coverage_artifact_name: core-interfaces-coverage-autograd-${{ matrix.python-version }}-numpy-1.26
+      python_version: ${{ matrix.python-version }}
+      pipeline_mode: ${{ inputs.pipeline_mode }}
+      install_numpy_1: true
+      install_jax: false
+      install_tensorflow: false
+      install_pytorch: false
+      install_pennylane_lightning_master: true
+      pytest_markers: autograd and not qcut and not finite-diff and not param-shift
+      disable_new_opmath: ${{ inputs.disable_new_opmath }}
+
+
+  jax-tests:
+    needs:
+      - setup-ci-load
+    strategy:
+      max-parallel: >-
+        ${{
+           fromJSON(needs.setup-ci-load.outputs.matrix-max-parallel).jax-tests
+           || fromJSON(needs.setup-ci-load.outputs.matrix-max-parallel).default
+         }}
+      matrix:
+        group: [1, 2, 3, 4, 5]
+        python-version: >-
+          ${{
+            fromJSON(needs.setup-ci-load.outputs.python-version).jax-tests
+            || fromJSON(needs.setup-ci-load.outputs.python-version).default
+           }}
+    uses: ./.github/workflows/unit-test.yml
+    with:
+      job_name: jax-tests (${{ matrix.group }}, ${{ matrix.python-version }}, numpy-1.26)
+      branch: ${{ inputs.branch }}
+      coverage_artifact_name: core-interfaces-coverage-jax-${{ matrix.python-version }}-${{ matrix.group }}-numpy-1.26
+      python_version: ${{ matrix.python-version }}
+      pipeline_mode: ${{ inputs.pipeline_mode }}
+      install_numpy_1: true
+      install_jax: true
+      install_tensorflow: false
+      install_pytorch: false
+      install_pennylane_lightning_master: true
+      pytest_markers: jax and not qcut and not finite-diff and not param-shift
+      pytest_additional_args: --dist=loadscope --splits 5 --group ${{ matrix.group }}
+      additional_pip_packages: pytest-split
+      requirements_file: ${{ github.event_name == 'schedule' && strategy.job-index == 0 && 'jax.txt' || '' }}
+      disable_new_opmath: ${{ inputs.disable_new_opmath }}
+
+
+  core-tests:
+    needs:
+      - setup-ci-load
+    strategy:
+      max-parallel: >-
+        ${{
+           fromJSON(needs.setup-ci-load.outputs.matrix-max-parallel).core-tests
+           || fromJSON(needs.setup-ci-load.outputs.matrix-max-parallel).default
+         }}
+      matrix:
+        group: [1, 2, 3, 4, 5]
+        python-version: >-
+          ${{
+            fromJSON(needs.setup-ci-load.outputs.python-version).core-tests
+            || fromJSON(needs.setup-ci-load.outputs.python-version).default
+           }}
+    uses: ./.github/workflows/unit-test.yml
+    with:
+      job_name: core-tests (${{ matrix.group }}, ${{ matrix.python-version }}, numpy-1.26)
+      branch: ${{ inputs.branch }}
+      coverage_artifact_name: core-interfaces-coverage-core-${{ matrix.python-version }}-${{ matrix.group }}-numpy-1.26
+      python_version: ${{ matrix.python-version }}
+      pipeline_mode: ${{ inputs.pipeline_mode }}
+      install_numpy_1: true
+      install_jax: false
+      install_tensorflow: false
+      install_pytorch: false
+      install_pennylane_lightning_master: true
+      pytest_markers: core and not qcut and not finite-diff and not param-shift
+      pytest_additional_args: --splits 5 --group ${{ matrix.group }}
+      additional_pip_packages: pytest-split
+      requirements_file: ${{ github.event_name == 'schedule' && strategy.job-index == 0 && 'core.txt' || '' }}
+      disable_new_opmath: ${{ inputs.disable_new_opmath }}
+
+
+  data-tests:
+    needs:
+      - setup-ci-load
+    strategy:
+      max-parallel: >-
+        ${{
+           fromJSON(needs.setup-ci-load.outputs.matrix-max-parallel).data-tests
+           || fromJSON(needs.setup-ci-load.outputs.matrix-max-parallel).default
+         }}
+      matrix:
+        python-version: >-
+          ${{
+            fromJSON(needs.setup-ci-load.outputs.python-version).data-tests
+            || fromJSON(needs.setup-ci-load.outputs.python-version).default
+           }}
+    uses: ./.github/workflows/unit-test.yml
+    with:
+      job_name: data-tests (${{ matrix.python-version }}, numpy-1.26)
+      branch: ${{ inputs.branch }}
+      coverage_artifact_name: data-coverage-${{ matrix.python-version }}-numpy-1.26
+      python_version: ${{ matrix.python-version }}
+      pipeline_mode: ${{ inputs.pipeline_mode }}
+      install_numpy_1: true
+      install_jax: false
+      install_tensorflow: false
+      install_pytorch: false
+      install_pennylane_lightning_master: true
+      pytest_markers: data
+      additional_pip_packages: h5py
+      disable_new_opmath: ${{ inputs.disable_new_opmath }}

.github/workflows/unit-test.yml

@@ -24,6 +24,11 @@ on:
         required: false
         type: string
         default: '3.10'
+      install_numpy_1:
+        description: Indicate if numpy 1 should be installed or not
+        required: false
+        type: boolean
+        default: false
       pipeline_mode:
         description: The pipeline mode can be unit-tests, benchmarks, or reference-benchmark
         required: false
@@ -158,6 +163,7 @@ jobs:
         uses: ./.github/workflows/install_deps
         with:
           python_version: ${{ inputs.python_version }}
+          install_numpy_1: ${{ inputs.install_numpy_1 }}
           install_pytorch: ${{ inputs.install_pytorch }}
           install_tensorflow: ${{ inputs.install_tensorflow }}
           install_jax: ${{ inputs.install_jax }}