Make transformers.utils.fx. _SUPPORTED_MODELS unique
Currently this tuple contains duplicates(e.g. `RobertaForMaskedLM` or `GPT2LMHeadModel`): `AlbertModel, AlbertForPreTraining, AlbertForMaskedLM, AlbertForMultipleChoice, AlbertForQuestionAnswering, AlbertForSequenceClassification, AlbertForTokenClassification, BertModel, BertForPreTraining, BertForNextSentencePrediction, BertForMaskedLM, BertLMHeadModel, BertForMultipleChoice, BertForQuestionAnswering, BertForSequenceClassification, BertForTokenClassification, DistilBertModel, DistilBertForMaskedLM, DistilBertForMaskedLM, DistilBertForMultipleChoice, DistilBertForQuestionAnswering, DistilBertForSequenceClassification, DistilBertForTokenClassification, MobileBertModel, MobileBertForPreTraining, MobileBertForNextSentencePrediction, MobileBertForMaskedLM, MobileBertForMultipleChoice, MobileBertForQuestionAnswering, MobileBertForSequenceClassification, MobileBertForTokenClassification, ElectraModel, ElectraForPreTraining, ElectraForMaskedLM, ElectraForCausalLM, ElectraForMultipleChoice, ElectraForQuestionAnswering, ElectraForSequenceClassification, ElectraForTokenClassification, MegatronBertModel, MegatronBertForPreTraining, MegatronBertForNextSentencePrediction, MegatronBertForMaskedLM, MegatronBertForCausalLM, MegatronBertForMultipleChoice, MegatronBertForQuestionAnswering, MegatronBertForSequenceClassification, MegatronBertForTokenClassification, GPT2Model, GPT2LMHeadModel, GPT2LMHeadModel, GPT2ForSequenceClassification, GPT2ForTokenClassification, GPTJModel, GPTJForCausalLM, GPTJForQuestionAnswering, GPTJForSequenceClassification, GPTNeoModel, GPTNeoForCausalLM, GPTNeoForSequenceClassification, T5Model, T5ForConditionalGeneration, T5ForConditionalGeneration, RobertaModel, RobertaForMaskedLM, RobertaForMaskedLM, RobertaForCausalLM, RobertaForMultipleChoice, RobertaForQuestionAnswering, RobertaForSequenceClassification, RobertaForTokenClassification, GPT2DoubleHeadsModel`

@michaelbenayoun 
_The documentation is not available anymore as the PR was closed or merged._<|||||>Merging with offline approval from @LysandreJik 