Add transformer api doc (#5029)

* update transformer doc

* Update transformers.md

* Update transformers.md

PaddleNLP/docs/transformers.md

-# 汇总paddlenlp.transformer下预训练模型
+# PaddleNLP transformer类预训练模型
+
+随着深度学习的发展，NLP领域涌现了一大批高质量的transformer类预训练模型，多次刷新各种NLP任务SOTA。PaddleNLP为用户提供了常用的BERT、ERNIE等预训练模型，让用户能够方便快捷的使用各种transformer类模型，完成自己所需的任务。
+
+## Transformer 类模型汇总
+
+
+| Model | Tokenizer| Supported Task| Pretrained Weight|
+|---|---|---|---|
+| [BERT](https://arxiv.org/abs/1810.04805) | BertTokenizer|BertModel<br> BertForQuestionAnswering<br> BertForSequenceClassification<br>BertForTokenClassification| `bert-base-uncased`<br> `bert-large-uncased` <br>`bert-base-multilingual-uncased` <br>`bert-base-cased`<br> `bert-base-chinese`<br> `bert-base-multilingual-cased`<br> `bert-large-cased`<br> `bert-wwm-chinese`<br> `bert-wwm-ext-chinese` |
+|[ELECTRA](https://arxiv.org/abs/2003.10555) |ElectraTokenizer| ElectraModel<br>ElectraForSequenceClassification<br>ElectraForTokenClassification<br>|`electra-small`<br> `electra-base`<br> `electra-large`<br> `chinese-electra-small`<br> `chinese-electra-base`<br>|
+|[ERNIE](https://arxiv.org/abs/1904.09223)|ErnieTokenizer<br>ErnieTinyTokenizer|ErnieModel<br> ErnieForQuestionAnswering<br> ErnieForSequenceClassification<br> ErnieForTokenClassification| `ernie-1.0`<br> `ernie-tiny`<br> `ernie-2.0-en`<br> `ernie-2.0-large-en`|
+|[RoBERTa](https://arxiv.org/abs/1907.11692)|RobertaTokenizer| RobertaModel<br>RobertaForQuestionAnswering<br>RobertaForSequenceClassification<br>RobertaForTokenClassification| `roberta-wwm-ext`<br> `roberta-wwm-ext-large`<br> `rbt3`<br> `rbtl3`|
+|[Transformer](https://arxiv.org/abs/1706.03762) |- | TransformerModel | - |