Fix model parallel definition in superclass
The `model_parallel` attribute should be obtainable from every class instance that has the `is_parallelized` class attribute. Otherwise the following line in the trainer crashes:

https://github.com/huggingface/transformers/blob/626116b7d76efef5137c3b4a92e64e3bb57a6882/src/transformers/trainer.py#L244

cc @stas00 @alexorona 