bert-base-uncased weights for BertForPreTraining
Hi,

According to the model card: https://huggingface.co/bert-base-uncased , these weights are obtained by training on two objectives (MLM and NSP) but the `config.json` has only the `BertForMaskedLM` tag. So when initializing `BertForPreTraining` model with `bert-base-uncased` weights, the following warning is printed:
`Some weights of BertForPreTraining were not initialized from the model checkpoint at bert-base-uncased and are newly initialized: ['cls.predictions.decoder.bias']`.

(My use-case would be to load `bert-base-uncased` weights and run the same pretraining process as the one described in the model card: https://huggingface.co/bert-base-uncased)

@LysandreJik 
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