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+*This model was published in HF papers on 2021-05-09 and contributed to Hugging Face Transformers on 2021-09-20.*
+
+# FNet
+
+
+## Overview
+
+The FNet model was proposed in [FNet: Mixing Tokens with Fourier Transforms](https://huggingface.co/papers/2105.03824) by
+James Lee-Thorp, Joshua Ainslie, Ilya Eckstein, Santiago Ontanon. The model replaces the self-attention layer in a BERT
+model with a fourier transform which returns only the real parts of the transform. The model is significantly faster
+than the BERT model because it has fewer parameters and is more memory efficient. The model achieves about 92-97%
+accuracy of BERT counterparts on GLUE benchmark, and trains much faster than the BERT model. The abstract from the
+paper is the following:
+
+*We show that Transformer encoder architectures can be sped up, with limited accuracy costs, by replacing the
+self-attention sublayers with simple linear transformations that "mix" input tokens. These linear mixers, along with
+standard nonlinearities in feed-forward layers, prove competent at modeling semantic relationships in several text
+classification tasks. Most surprisingly, we find that replacing the self-attention sublayer in a Transformer encoder
+with a standard, unparameterized Fourier Transform achieves 92-97% of the accuracy of BERT counterparts on the GLUE
+benchmark, but trains 80% faster on GPUs and 70% faster on TPUs at standard 512 input lengths. At longer input lengths,
+our FNet model is significantly faster: when compared to the "efficient" Transformers on the Long Range Arena
+benchmark, FNet matches the accuracy of the most accurate models, while outpacing the fastest models across all
+sequence lengths on GPUs (and across relatively shorter lengths on TPUs). Finally, FNet has a light memory footprint
+and is particularly efficient at smaller model sizes; for a fixed speed and accuracy budget, small FNet models
+outperform Transformer counterparts.*
+
+This model was contributed by [gchhablani](https://huggingface.co/gchhablani). The original code can be found [here](https://github.com/google-research/google-research/tree/master/f_net).
+
+## Usage tips
+
+The model was trained without an attention mask as it is based on Fourier Transform. The model was trained with
+maximum sequence length 512 which includes pad tokens. Hence, it is highly recommended to use the same maximum
+sequence length for fine-tuning and inference.
+
+## Resources
+
+- [Text classification task guide](../tasks/sequence_classification)
+- [Token classification task guide](../tasks/token_classification)
+- [Question answering task guide](../tasks/question_answering)
+- [Masked language modeling task guide](../tasks/masked_language_modeling)
+- [Multiple choice task guide](../tasks/multiple_choice)
+
+## FNetConfig
+
+[[autodoc]] FNetConfig
+
+## FNetTokenizer
+
+[[autodoc]] FNetTokenizer
+    - get_special_tokens_mask
+    - save_vocabulary
+
+## FNetTokenizerFast
+
+[[autodoc]] FNetTokenizerFast
+
+## FNetModel
+
+[[autodoc]] FNetModel
+    - forward
+
+## FNetForPreTraining
+
+[[autodoc]] FNetForPreTraining
+    - forward
+
+## FNetForMaskedLM
+
+[[autodoc]] FNetForMaskedLM
+    - forward
+
+## FNetForNextSentencePrediction
+
+[[autodoc]] FNetForNextSentencePrediction
+    - forward
+
+## FNetForSequenceClassification
+
+[[autodoc]] FNetForSequenceClassification
+    - forward
+
+## FNetForMultipleChoice
+
+[[autodoc]] FNetForMultipleChoice
+    - forward
+
+## FNetForTokenClassification
+
+[[autodoc]] FNetForTokenClassification
+    - forward
+
+## FNetForQuestionAnswering
+
+[[autodoc]] FNetForQuestionAnswering
+    - forward