Migrate model card from transformers-repo

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Original file history: https://github.com/huggingface/transformers/commits/master/model_cards/google/reformer-enwik8/README.md

README.md

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+## Reformer Language model on character level and trained on enwik8. 
+
+*enwik8* is a dataset based on Wikipedia and is often used to measure the model's ability to *compress* data, *e.g.* in 
+the scope of the *Hutter prize*: https://en.wikipedia.org/wiki/Hutter_Prize.
+
+`reformer-enwik8` was pretrained on the first 90M chars of *enwik8* whereas the text was chunked into batches of size 65536 chars (=2^16).
+The model's weights were taken from https://console.cloud.google.com/storage/browser/trax-ml/reformer/enwik8 and converted 
+to Hugging Face's PyTorch ReformerLM model `ReformerModelWithLMHead`.
+
+The model is a language model that operates on characters. 
+Therefore, this model does not need a tokenizer. The following function can instead be used for **encoding** and **decoding**:
+
+```python
+import torch
+
+# Encoding
+def encode(list_of_strings, pad_token_id=0):
+    max_length = max([len(string) for string in list_of_strings])
+
+    # create emtpy tensors
+    attention_masks = torch.zeros((len(list_of_strings), max_length), dtype=torch.long)
+    input_ids = torch.full((len(list_of_strings), max_length), pad_token_id, dtype=torch.long)
+
+    for idx, string in enumerate(list_of_strings):
+        # make sure string is in byte format
+        if not isinstance(string, bytes):
+            string = str.encode(string)
+
+        input_ids[idx, :len(string)] = torch.tensor([x + 2 for x in string])
+        attention_masks[idx, :len(string)] = 1
+
+    return input_ids, attention_masks
+    
+# Decoding
+def decode(outputs_ids):
+    decoded_outputs = []
+    for output_ids in outputs_ids.tolist():
+        # transform id back to char IDs < 2 are simply transformed to ""
+        decoded_outputs.append("".join([chr(x - 2) if x > 1 else "" for x in output_ids]))
+    return decoded_outputs
+```
+
+Text can be generated as follows:
+
+```python
+from transformers import ReformerModelWithLMHead
+
+model = ReformerModelWithLMHead.from_pretrained("google/reformer-enwik8")
+encoded, attention_masks = encode(["In 1965, Brooks left IBM to found the Department of"])
+decode(model.generate(encoded, do_sample=True, max_length=150))
+
+# gives:
+# In 1965, Brooks left IBM to found the Department of Journalism in 1968. IBM had jurisdiction himself in 1980, while Brooks resolved, nevertheless thro
+
+```
+
+***Note***: Language generation using `ReformerModelWithLMHead` is not optimized yet and is rather slow.