[PretrainedModel] add tie_weights to init
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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When someone wants to pretrain a model from scratch with `config.tie_word_embeddings=True`, one would expect that even 
when doing:

```python 
model = BertModel(BertConfig())
```

that the word embedding weights are tied. However this is not the case at the moment. This PR fixes it


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Comments:
Actually the init takes care of this so no need for this PR