Remove OVBaseModelForSeq2SeqLM (huggingface#1278)

* remove unused OVBaseModelForSeq2SeqLM

* iterate on ovsubmodels

* fix style

* remove OVBaseModelForSeq2Seq

* fix style

* fixes

* fix

* remove unecessary encoder decoder attribute

* uniformize

* style

* fixes

* style

* adapt tests

* fixes quantization

* clean

* clean

* style

* add test

* style

* fix typo

* clean

* fix

* fix

* make style

Author: echarlaix