Migrate model card from transformers-repo

Read announcement at https://discuss.huggingface.co/t/announcement-all-model-cards-will-be-migrated-to-hf-co-model-repos/2755
Original file history: https://github.com/huggingface/transformers/commits/master/model_cards/funnel-transformer/small-base/README.md

README.md

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+---
+language: en
+license: apache-2.0
+datasets:
+- bookcorpus
+- wikipedia
+- gigaword
+---
+
+# Funnel Transformer small model (B4-4-4 without decoder)
+
+Pretrained model on English language using a similar objective objective as [ELECTRA](https://huggingface.co/transformers/model_doc/electra.html). It was introduced in
+[this paper](https://arxiv.org/pdf/2006.03236.pdf) and first released in
+[this repository](https://github.com/laiguokun/Funnel-Transformer). This model is uncased: it does not make a difference
+between english and English.
+
+Disclaimer: The team releasing Funnel Transformer did not write a model card for this model so this model card has been
+written by the Hugging Face team.
+
+## Model description
+
+Funnel Transformer is a transformers model pretrained on a large corpus of English data in a self-supervised fashion. This means it
+was pretrained on the raw texts only, with no humans labelling them in any way (which is why it can use lots of
+publicly available data) with an automatic process to generate inputs and labels from those texts. 
+
+More precisely, a small language model corrupts the input texts and serves as a generator of inputs for this model, and
+the pretraining objective is to predict which token is an original and which one has been replaced, a bit like a GAN training.
+
+This way, the model learns an inner representation of the English language that can then be used to extract features
+useful for downstream tasks: if you have a dataset of labeled sentences for instance, you can train a standard
+classifier using the features produced by the BERT model as inputs.
+
+**Note:** This model does not contain the decoder, so it ouputs hidden states that have a sequence length of one fourth
+of the inputs. It's good to use for tasks requiring a summary of the sentence (like sentence classification) but not if
+you need one input per initial token. You should use the `small` model in that case.
+
+## Intended uses & limitations
+
+You can use the raw model to extract a vector representation of a given text, but it's mostly intended to
+be fine-tuned on a downstream task. See the [model hub](https://huggingface.co/models?filter=funnel-transformer) to look for
+fine-tuned versions on a task that interests you.
+
+Note that this model is primarily aimed at being fine-tuned on tasks that use the whole sentence (potentially masked)
+to make decisions, such as sequence classification, token classification or question answering. For tasks such as text
+generation you should look at model like GPT2.
+
+### How to use
+
+
+Here is how to use this model to get the features of a given text in PyTorch:
+
+```python
+from transformers import FunnelTokenizer, FunnelBaseModel
+tokenizer = FunnelTokenizer.from_pretrained("funnel-transformer/small-base")
+model = FunnelBaseModel.from_pretrained("funnel-transformer/small-base")
+text = "Replace me by any text you'd like."
+encoded_input = tokenizer(text, return_tensors='pt')
+output = model(**encoded_input)
+```
+
+and in TensorFlow:
+
+```python
+from transformers import FunnelTokenizer, TFFunnelBaseModel
+tokenizer = FunnelTokenizer.from_pretrained("funnel-transformer/small-base")
+model = TFFunnelBaseModel.from_pretrained("funnel-transformer/small-base")
+text = "Replace me by any text you'd like."
+encoded_input = tokenizer(text, return_tensors='tf')
+output = model(encoded_input)
+```
+
+## Training data
+
+The BERT model was pretrained on:
+- [BookCorpus](https://yknzhu.wixsite.com/mbweb), a dataset consisting of 11,038 unpublished books,
+- [English Wikipedia](https://en.wikipedia.org/wiki/English_Wikipedia) (excluding lists, tables and headers),
+- [Clue Web](https://lemurproject.org/clueweb12/), a dataset of 733,019,372 English web pages,
+- [GigaWord](https://catalog.ldc.upenn.edu/LDC2011T07), an archive of newswire text data,
+- [Common Crawl](https://commoncrawl.org/), a dataset of raw web pages.
+
+
+### BibTeX entry and citation info
+
+```bibtex
+@misc{dai2020funneltransformer,
+    title={Funnel-Transformer: Filtering out Sequential Redundancy for Efficient Language Processing},
+    author={Zihang Dai and Guokun Lai and Yiming Yang and Quoc V. Le},
+    year={2020},
+    eprint={2006.03236},
+    archivePrefix={arXiv},
+    primaryClass={cs.LG}
+}
+```
+