input: Write a python function to retrieve all unique keys from a nested dictionary.
  assert get_all_keys({"key1": [1, 2], "key2": [2, 3], "key3": {"key4": [3, 4]}}) == {"key1", "key2", "key3", "key4"}
  assert get_all_keys({"a": {"b": {"c": {"d": 1}}}}) == {"a", "b", "c", "d"}
  assert get_all_keys({"x": 1, "y": {"z": {"w": 2}}, "v": [1, 2, 3]}) == {"x", "y", "z", "w", "v"}
output: def get_all_keys(nested_dict):
      keys = set()
      
      def retrieve_keys(d):
          for key, value in d.items():
              keys.add(key)
              if isinstance(value, dict):
                  retrieve_keys(value)
      
      retrieve_keys(nested_dict)
      return keys