Cannot pickle 'weakref' object when using Tensorflow with Multiprocessing

I would like to train several neural networks at the same time, and I'm trying to use the multiprocessing module so that each network can be trained in a separate process, but I met an issue. When I ran the demo code below (as apply_async function does not give a hint on errors, I temporarily changed it to apply function):

import tensorflow as tf
import multiprocessing as mp


class SeqModel(tf.keras.Sequential):
    def __init__(self, input_size, hidden_sizes, output_size):
        super().__init__()
        self.add(tf.keras.layers.Dense(hidden_sizes[0], activation="relu", input_shape=(input_size,)))
        for hidden_size in hidden_sizes[1:]: self.add(tf.keras.layers.Dense(hidden_size, activation="relu"))
        if output_size is not None: self.add(tf.keras.layers.Dense(output_size))


class Partition:
    def __init__(self, partition_id):
        self.partition_id = partition_id
        self.model = None

    def initialization(self):
        self.model = SeqModel(10,[10,10],10)

    def test(self):
        print(f'partition {self.partition_id} testing...')


def func():
    partition_list = [Partition(i) for i in range(4)]

    for partition in partition_list: partition.initialization()

    p = mp.Pool(4)
    for partition in partition_list:
        p.apply(partition.test)
    p.close()
    p.join()


if __name__ == '__main__':
    func()

I got the following error:

Traceback (most recent call last):
  File "C:/Users/Administrator/Dropbox (ASU)/Work/Traffic State Estimation/traffic state estimation/dataset/mp/mp_net.py", line 43, in <module>
    func()
  File "C:/Users/Administrator/Dropbox (ASU)/Work/Traffic State Estimation/traffic state estimation/dataset/mp/mp_net.py", line 37, in func
    p.apply(partition.test)
  File "C:\Users\Administrator\AppData\Local\Programs\Python\Python38\lib\multiprocessing\pool.py", line 357, in apply
    return self.apply_async(func, args, kwds).get()
  File "C:\Users\Administrator\AppData\Local\Programs\Python\Python38\lib\multiprocessing\pool.py", line 771, in get
    raise self._value
  File "C:\Users\Administrator\AppData\Local\Programs\Python\Python38\lib\multiprocessing\pool.py", line 537, in _handle_tasks
    put(task)
  File "C:\Users\Administrator\AppData\Local\Programs\Python\Python38\lib\multiprocessing\connection.py", line 206, in send
    self._send_bytes(_ForkingPickler.dumps(obj))
  File "C:\Users\Administrator\AppData\Local\Programs\Python\Python38\lib\multiprocessing\reduction.py", line 51, in dumps
    cls(buf, protocol).dump(obj)
TypeError: cannot pickle 'weakref' object

If I do not do partition initialization (no SeqModel involved in Partition instances), the code runs without problem. Does that mean I cannot use tf Models in child processes?