add `classifier_dropout` to classification heads
PR for #12781 and maybe a fix for #12792

# Logbook
- added to Electra: `SequenceClassification` and `TokenClassification`
- added to BERT: `SequenceClassification` and `TokenClassification`
- added to RoBERTa: `SequenceClassification` and `TokenClassification`
- added to big_bird: `SequenceClassification` and `TokenClassification`
- added to mobilebert: `SequenceClassification` and `TokenClassification`
- added to reformer: `SequenceClassification` <s>and `TokenClassification`</s>
- added to ConvBERT: `SequenceClassification` and `TokenClassification`
- added to albert: `SequenceClassification` and `TokenClassification`
@sgugger should `classifier_dropout` be only added for `<model_name>ForSequenceClassification`
or also to `<model_name>ForTokenClassification`?<|||||>It should be used for both, yes.<|||||>This is still WIP - I want to add more models. Please do not merge yet ...<|||||>I am done with this PR. It is ready for review.
See logbook above for changed model types.