## End-to-End Training of Neural Retrievers for Open-Domain Question Answering
Below we present the steps to run unsupervised and supervised trainining and evaluation of the retriever for [open domain question answering](https://arxiv.org/abs/2101.00408).
## Retriever Training
#### Unsupervised pretraining
1. Use `tools/preprocess_data.py` to preprocess the dataset for Inverse Cloze Task (ICT), which we call unsupervised pretraining. This script takes as input a corpus in loose JSON format and creates fixed-size blocks of text as the fundamental units of data. For a corpus like Wikipedia, this will mean multiple sentences per block and multiple blocks per document. Run [`tools/preprocess_data.py`](../../tools/preprocess_data.py) to construct one or more indexed datasets with the `--split-sentences` argument to make sentences the basic unit. We construct two datasets, one with the title of every document and another with the body.
<pre>
python tools/preprocess_data.py \
--input /path/to/corpus.json \
--json-keys text title \
--split-sentences \
--tokenizer-type BertWordPieceLowerCase \
--vocab-file /path/to/vocab.txt \
--output-prefix corpus_indexed \
--workers 10
</pre>
2. The [`examples/pretrain_ict.sh`](../../examples/pretrain_ict.sh) script runs a single GPU 217M parameter biencoder model for ICT retriever training. Single GPU training is primarily intended for debugging purposes, as the code is developed for distributed training. The script uses a pretrained BERT model and we use a total of batch size of 4096 for the ICT training.
3. Evaluate the pretrained ICT model using [`examples/evaluate_retriever_nq.sh`](../../examples/evaluate_retriever_nq.sh) for [Google's Natural Questions Open dataset](https://arxiv.org/pdf/1906.00300.pdf).
#### Supervised finetuning
1. Use the above pretrained ICT model to finetune using [Google's Natural Questions Open dataset](https://github.com/google-research/language/tree/master/language/orqa). The script [`examples/finetune_retriever_distributed.sh`](../../examples/finetune_retriever_distributed.sh) provides an example for how to perform the training. Our finetuning process includes retriever score scaling and longer training (80 epochs) on top [DPR training](https://arxiv.org/abs/2004.04906).
2. Evaluate the finetuned model using the same evaluation script as mentioned above for the unsupervised model.
More details on the retriever are available in [our paper](https://arxiv.org/abs/2101.00408).
@@ -20,7 +20,7 @@ python blacklist_urls.py <path to the dowloaded deduplicated URLs> <filename for
4. Merge the contents into one loose json file with 1 json per newline of the format `{'text': text, 'url': unique_url}`. It is important for the url to be unique.
# Prepare the data for GPT-2 training:
# Prepare the data for GPT training:
1. Perform ftfy, english detection and remove documents with less than 128 tokens. This step can be sharded and run on shards.
To deduplicate the downstream tasks (e.g. lambada, squad) from the training dataset, we run the following command.
```
python filter_ngrams.py --tasks <name of he task, e.g. lambada, squad> --dedup-dataset <training dataset to deduplicate> <json key> --output <output training dataset>
python filter_ngrams.py --tasks <name of the task, e.g. lambada, squad> --dedup-dataset <training dataset to deduplicate> <json key> --output <output training dataset>
```
We use 13-grams by default for the deduplication. When we find a 13-gram match in a training document, we split the document into two pieces and remove the 13-gram along with 200 characters from the both side of the 13-gram. We also remove any splitted document with less than 200 characters or if a document got splitted more than 10 times. These parameters can be changed using corresponding arguments.
