[Tests] Add Common Test for Training + Fix a couple of bugs
Author: patrickvonplaten
Pull request: yes
Labels: name: model card

# What does this PR do?

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This PR adds an aggressive test to check that all models that should be trainable can perform a backward pass of their loss output. In addition, a test for training with gradient checkpointing is added as well. The motivation comes from this error: https://github.com/huggingface/transformers/pull/7562#issuecomment-723887221  - the PR introduced broke gradient checkpointing without any test noticing it. 

To make the test applicable for all models, some `ModelTests` have to overwrite the `_prepare_for_class` function.
In addition, some cleaning was done `ForPretraining` was renamed to `ForPreTraining`; an `AutoModelForNextSentencePredicition` was added, ...




## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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 albert, bert, XLM: @LysandreJik
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 nlp datasets: [different repo](https://github.com/huggingface/nlp)
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