input: Write a python function to create a MultiValueDict class that can add and retrieve multiple values for a single key.
  assert mvd.get('a') == None
  mvd.add('a', 1)
  mvd.add('a', 2)
  assert mvd.get('a') == [1, 2]
  mvd.add('b', 3)
  assert mvd.get('b') == [3]
  mvd.add('a', 3)
  assert mvd.get('a') == [1, 2, 3]
output: def MultiValueDict():
      class MultiValueDict:
          def __init__(self):
              self.data = {}
  
          def add(self, key, value):
              if key not in self.data:
                  self.data[key] = []
              self.data[key].append(value)
  
          def get(self, key):
              return self.data.get(key, None)
  
      return MultiValueDict()
  
  # Example usage:
  mvd = MultiValueDict()