Move singleton to common directory (#10935)

### What problem does this PR solve?

As title

### Type of change

- [x] Refactoring

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

test/unit_test/common/test_decorator.py

@@ -0,0 +1,79 @@
+#
+#  Copyright 2025 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+
+from common.decorator import singleton
+
+
+# Test class for demonstration
+@singleton
+class TestClass:
+    def __init__(self):
+        self.counter = 0
+
+    def increment(self):
+        self.counter += 1
+        return self.counter
+
+
+# Test cases
+class TestSingleton:
+
+    def test_state_persistence(self):
+        """Test that instance state persists across multiple calls"""
+        instance1 = TestClass()
+        instance1.increment()
+        instance1.increment()
+
+        instance2 = TestClass()
+        assert instance2.counter == 2  # State should persist
+
+    def test_multiple_calls_consistency(self):
+        """Test consistency across multiple calls"""
+        instances = [TestClass() for _ in range(5)]
+
+        # All references should point to the same object
+        first_instance = instances[0]
+        for instance in instances:
+            assert instance is first_instance
+
+    def test_instance_methods_work(self):
+        """Test that instance methods work correctly"""
+        instance = TestClass()
+
+        # Test method calls
+        result1 = instance.increment()
+        result2 = instance.increment()
+
+        assert result1 == 3
+        assert result2 == 4
+        assert instance.counter == 4
+
+
+# Test decorator itself
+def test_singleton_decorator_returns_callable():
+    """Test that the decorator returns a callable"""
+
+    class PlainClass:
+        pass
+
+    decorated_class = singleton(PlainClass)
+
+    # Should return a function
+    assert callable(decorated_class)
+
+    # Calling should return an instance of PlainClass
+    instance = decorated_class()
+    assert isinstance(instance, PlainClass)