Add AutoModelForPreTraining
Add `AutoModelForPretraining` and `TFAutoModelForPretraining` classes which will load the full model used for pretraining (guarantee we should have all the pre-trained weights).

This class can be used for instance to convert between an original PyTorch and a TF2.0 models while being sure that all the pretrained weights are converted:

```python
# PyTorch => TF 2.0 (save TF 2.0 weights from PT weights)
tf_model = TFAutoModelForPretraining.from_pretrained('my-model', from_pt=True)
tf_model.save_pretrained()

# TF 2.0 => PyTorch (save PT weights from TF 2.0 weights)
pt_model = AutoModelForPretraining.from_pretrained('my-model', from_tf=True)
pt_model.save_pretrained()
```