include to_json_string

[jcrangel]

To fix :

    trainer.train()
  File "/home/julio/anaconda3/envs/mamba/lib/python3.10/site-packages/transformers/trainer.py", line 1555, in train
    return inner_training_loop(
  File "/home/julio/anaconda3/envs/mamba/lib/python3.10/site-packages/transformers/trainer.py", line 1789, in _inner_training_loop
    self.control = self.callback_handler.on_train_begin(args, self.state, self.control)
  File "/home/julio/anaconda3/envs/mamba/lib/python3.10/site-packages/transformers/trainer_callback.py", line 363, in on_train_begin
    return self.call_event("on_train_begin", args, state, control)
  File "/home/julio/anaconda3/envs/mamba/lib/python3.10/site-packages/transformers/trainer_callback.py", line 407, in call_event
    result = getattr(callback, event)(
  File "/home/julio/anaconda3/envs/mamba/lib/python3.10/site-packages/transformers/integrations/integration_utils.py", line 631, in on_train_begin
    model_config_json = model.config.to_json_string()
AttributeError: 'MambaConfig' object has no attribute 'to_json_string'

when training with hugging face and saving the model.