Merge pull request optuna#5067 from nabenabe0928/reduce-tests-mpi-to-…

…oldest-and-latest

Reduce `tests-mpi` to the oldest and latest Python versions

.github/workflows/tests-mpi.yml

@@ -1,6 +1,7 @@
 name: Tests (MPI)
 
 on:
+  workflow_dispatch:
   push:
     branches:
       - master
@@ -13,13 +14,29 @@ concurrency:
   cancel-in-progress: true
 
 jobs:
+  define-matrix:
+    runs-on: ubuntu-latest
+    outputs:
+      matrix: ${{ steps.set-matrix.outputs.matrix }}
+    steps:
+    - name: Define workflow matrix
+      id: set-matrix
+      run: |
+        if [ "${{ github.event_name }}" = "schedule" ] || [ "${{ github.event_name }}" = "workflow_dispatch" ] ; then
+          echo "Schedule tests (run all the versions)"
+          echo "matrix={\"python-version\":[\"3.8\", \"3.9\", \"3.10\", \"3.11\"]}" >> $GITHUB_OUTPUT
+        else
+          echo "PR push tests (run only the oldest and the latest versions)"
+          echo "matrix={\"python-version\":[\"3.8\", \"3.11\"]}" >> $GITHUB_OUTPUT
+        fi
+
   tests-mpi:
     if: (github.event_name == 'schedule' && github.repository == 'optuna/optuna') || (github.event_name != 'schedule')
+    needs: define-matrix
     runs-on: ubuntu-latest
 
     strategy:
-      matrix:
-        python-version: ['3.8', '3.9', '3.10', '3.11']
+      matrix: ${{fromJson(needs.define-matrix.outputs.matrix)}}
 
     steps:
     - name: Checkout