input: Write a python function to add a dense layer to a Keras model.
  assert add_dense_layer(model, 64, 'relu', 100) == model
  assert add_dense_layer(model, 128, 'sigmoid', 64) == model
  assert isinstance(add_dense_layer(model, 256, 'tanh', 128), keras.models.Sequential)
output: def add_dense_layer(model, units, activation, input_dim=None):
      from tensorflow.keras.layers import Dense
      
      if input_dim is not None:
          model.add(Dense(units=units, activation=activation, input_dim=input_dim))
      else:
          model.add(Dense(units=units, activation=activation))
      
      return model