Fixed #25833 -- Added support for non-atomic migrations.

Added the Migration.atomic attribute which can be set to False
for non-atomic migrations.

tests/migrations/test_migrations_atomic_operation/0001_initial.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+def raise_error(apps, schema_editor):
+    # Test atomic operation in non-atomic migration is wrapped in transaction
+    Editor = apps.get_model('migrations', 'Editor')
+    Editor.objects.create(name='Test Editor')
+    raise RuntimeError('Abort migration')
+
+
+class Migration(migrations.Migration):
+    atomic = False
+
+    operations = [
+        migrations.CreateModel(
+            "Editor",
+            [
+                ("name", models.CharField(primary_key=True, max_length=255)),
+            ],
+        ),
+        migrations.RunPython(raise_error, atomic=True),
+    ]