Migrate model card from transformers-repo

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Original file history: https://github.com/huggingface/transformers/commits/master/model_cards/gpt2-README.md

README.md

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+---
+language: en
+tags:
+- exbert
+
+license: mit
+---
+
+
+# GPT-2
+
+Test the whole generation capabilities here: https://transformer.huggingface.co/doc/gpt2-large
+
+Pretrained model on English language using a causal language modeling (CLM) objective. It was introduced in
+[this paper](https://d4mucfpksywv.cloudfront.net/better-language-models/language_models_are_unsupervised_multitask_learners.pdf)
+and first released at [this page](https://openai.com/blog/better-language-models/).
+
+Disclaimer: The team releasing GPT-2 also wrote a
+[model card](https://github.com/openai/gpt-2/blob/master/model_card.md) for their model. Content from this model card
+has been written by the Hugging Face team to complete the information they provided and give specific examples of bias.
+
+## Model description
+
+GPT-2 is a transformers model pretrained on a very 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,
+it was trained to guess the next word in sentences.
+
+More precisely, inputs are sequences of continuous text of a certain length and the targets are the same sequence,
+shifted one token (word or piece of word) to the right. The model uses internally a mask-mechanism to make sure the
+predictions for the token `i` only uses the inputs from `1` to `i` but not the future tokens.
+
+This way, the model learns an inner representation of the English language that can then be used to extract features
+useful for downstream tasks. The model is best at what it was pretrained for however, which is generating texts from a
+prompt.
+
+## Intended uses & limitations
+
+You can use the raw model for text generation or fine-tune it to a downstream task. See the
+[model hub](https://huggingface.co/models?filter=gpt2) to look for fine-tuned versions on a task that interests you.
+
+### How to use
+
+You can use this model directly with a pipeline for text generation. Since the generation relies on some randomness, we
+set a seed for reproducibility:
+
+```python
+>>> from transformers import pipeline, set_seed
+>>> generator = pipeline('text-generation', model='gpt2')
+>>> set_seed(42)
+>>> generator("Hello, I'm a language model,", max_length=30, num_return_sequences=5)
+
+[{'generated_text': "Hello, I'm a language model, a language for thinking, a language for expressing thoughts."},
+ {'generated_text': "Hello, I'm a language model, a compiler, a compiler library, I just want to know how I build this kind of stuff. I don"},
+ {'generated_text': "Hello, I'm a language model, and also have more than a few of your own, but I understand that they're going to need some help"},
+ {'generated_text': "Hello, I'm a language model, a system model. I want to know my language so that it might be more interesting, more user-friendly"},
+ {'generated_text': 'Hello, I\'m a language model, not a language model"\n\nThe concept of "no-tricks" comes in handy later with new'}]
+```
+
+Here is how to use this model to get the features of a given text in PyTorch:
+
+```python
+from transformers import GPT2Tokenizer, GPT2Model
+tokenizer = GPT2Tokenizer.from_pretrained('gpt2')
+model = GPT2Model.from_pretrained('gpt2')
+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 GPT2Tokenizer, TFGPT2Model
+tokenizer = GPT2Tokenizer.from_pretrained('gpt2')
+model = TFGPT2Model.from_pretrained('gpt2')
+text = "Replace me by any text you'd like."
+encoded_input = tokenizer(text, return_tensors='tf')
+output = model(encoded_input)
+```
+
+### Limitations and bias
+
+The training data used for this model has not been released as a dataset one can browse. We know it contains a lot of
+unfiltered content from the internet, which is far from neutral. As the openAI team themselves point out in their
+[model card](https://github.com/openai/gpt-2/blob/master/model_card.md#out-of-scope-use-cases):
+
+> Because large-scale language models like GPT-2 do not distinguish fact from fiction, we don’t support use-cases
+> that require the generated text to be true.
+>
+> Additionally, language models like GPT-2 reflect the biases inherent to the systems they were trained on, so we do
+> not recommend that they be deployed into systems that interact with humans > unless the deployers first carry out a
+> study of biases relevant to the intended use-case. We found no statistically significant difference in gender, race,
+> and religious bias probes between 774M and 1.5B, implying all versions of GPT-2 should be approached with similar
+> levels of caution around use cases that are sensitive to biases around human attributes.
+
+Here's an example of how the model can have biased predictions:
+
+```python
+>>> from transformers import pipeline, set_seed
+>>> generator = pipeline('text-generation', model='gpt2')
+>>> set_seed(42)
+>>> generator("The White man worked as a", max_length=10, num_return_sequences=5)
+
+[{'generated_text': 'The White man worked as a mannequin for'},
+ {'generated_text': 'The White man worked as a maniser of the'},
+ {'generated_text': 'The White man worked as a bus conductor by day'},
+ {'generated_text': 'The White man worked as a plumber at the'},
+ {'generated_text': 'The White man worked as a journalist. He had'}]
+
+>>> set_seed(42)
+>>> generator("The Black man worked as a", max_length=10, num_return_sequences=5)
+
+[{'generated_text': 'The Black man worked as a man at a restaurant'},
+ {'generated_text': 'The Black man worked as a car salesman in a'},
+ {'generated_text': 'The Black man worked as a police sergeant at the'},
+ {'generated_text': 'The Black man worked as a man-eating monster'},
+ {'generated_text': 'The Black man worked as a slave, and was'}]
+```
+
+This bias will also affect all fine-tuned versions of this model.
+
+## Training data
+
+The OpenAI team wanted to train this model on a corpus as large as possible. To build it, they scraped all the web
+pages from outbound links on Reddit which received at least 3 karma. Note that all Wikipedia pages were removed from
+this dataset, so the model was not trained on any part of Wikipedia. The resulting dataset (called WebText) weights
+40GB of texts but has not been publicly released. You can find a list of the top 1,000 domains present in WebText
+[here](https://github.com/openai/gpt-2/blob/master/domains.txt).
+
+## Training procedure
+
+### Preprocessing
+
+The texts are tokenized using a byte-level version of Byte Pair Encoding (BPE) (for unicode characters) and a
+vocabulary size of 50,257. The inputs are sequences of 1024 consecutive tokens.
+
+The larger model was trained on 256 cloud TPU v3 cores. The training duration was not disclosed, nor were the exact
+details of training.
+
+## Evaluation results
+
+The model achieves the following results without any fine-tuning (zero-shot):
+
+| Dataset  | LAMBADA | LAMBADA | CBT-CN | CBT-NE | WikiText2 | PTB    | enwiki8 | text8  | WikiText103 | 1BW   |
+|:--------:|:-------:|:-------:|:------:|:------:|:---------:|:------:|:-------:|:------:|:-----------:|:-----:|
+| (metric) | (PPL)   | (ACC)   | (ACC)  | (ACC)  | (PPL)     | (PPL)  | (BPB)   | (BPC)  | (PPL)       | (PPL) |
+|          | 35.13   | 45.99   | 87.65  | 83.4   | 29.41     | 65.85  | 1.16    | 1,17   | 37.50       | 75.20 |
+
+
+### BibTeX entry and citation info
+
+```bibtex
+@article{radford2019language,
+  title={Language Models are Unsupervised Multitask Learners},
+  author={Radford, Alec and Wu, Jeff and Child, Rewon and Luan, David and Amodei, Dario and Sutskever, Ilya},
+  year={2019}
+}
+```
+
+<a href="https://huggingface.co/exbert/?model=gpt2">
+	<img width="300px" src="https://cdn-media.huggingface.co/exbert/button.png">
+</a>