Singleton Factory

Create a class `SingletonFactory` that manages singleton instances. The class should have: - An `__init__` method that initializes an empty internal dictionary. - A method `get_instance(self, key, *args, **kwargs)` that: - If `key` already exists in the internal dictionary, returns the existing instance without calling the constructor. - Otherwise, creates a new instance of the class `key` (which is a class object, e.g., `str`, `list`, or a custom class), passing `*args` and `**kwargs` to its constructor, stores it in the dictionary, and returns it. - A method `instance_count(self)` that returns the number of distinct keys stored. The factory should ensure that for the same key, the same instance is returned every time. Implement the `SingletonFactory` class with the specified methods. The tests will instantiate the factory and use helper functions to verify behavior.

>>> class Counter:
...     def __init__(self, start=0):
...         self.value = start
...     def increment(self):
...         self.value += 1
>>> f = SingletonFactory()
>>> c1 = f.get_instance(Counter, 10)
>>> c2 = f.get_instance(Counter)
>>> c1 is c2
True
>>> c1.increment()
>>> c1.value
11
>>> c2.value
11
>>> f.instance_count()
1
>>> f.get_instance(list)
[]
>>> f.instance_count()
2