Support for `BertModel`, `BartModel` (`AutoModel`)

[kainoj]

Until now: the library supports pruning for BertFor* (AutoModelFor*). Now: you can load both BertFor* and BertModel (or just use AutoModel).

What's new: I upgraded the pattern prefix regexp so it optionally captures bert. (or model. for Bart).

• Pattern prefixes for BertFor* start with bert.encoder.layer..
• Pattern prefixes for vanila BertModel start with just encoder.layer..