Although the powerful performance of large language models in the NLP field is amazing, the negative costs it brings are also serious, such as training is too expensive and difficult to update. , and it is difficult to deal with long-tail knowledge.
And language models usually use a softmax layer containing a limited vocabulary in the prediction layer, which basically does not output rare words or phrases, which greatly limits the expression ability of the model.
In order to solve the long-tail problem of the model, scholars from the University of Washington, Meta AI and the Allen Institute for Artificial Intelligence recently jointly proposed the first "NonParametric Masked language model" (NonParametric Masked language model, NPM), which replaces the softmax output by referring to the non-parametric distribution of each phrase in the corpus.
Paper link: https://arxiv.org/abs/2212.01349
Code link: https://github.com/facebookresearch/NPM
NPM can be effectively trained by contrastive objective and intra-batch approximation of retrieving the complete corpus.
The researchers conducted a zero-shot evaluation on nine closed tasks and seven open tasks, including spatiotemporal transformation and word-level translation tasks that emphasize the need to predict new facts or rare phrases.
The results show that regardless of whether retrieval and generation methods are used, NPM is significantly better than larger parameter models. For example, GPT-3 with 500 times more parameters and OPT 13B with 37 times more performance are much better. , and NPM is particularly good at handling rare patterns (word meanings or facts) and predicting rare or barely seen words (such as non-Latin scripts).
Although this problem can be alleviated by combining some existing retrieval-and-generate related work, the final prediction part of these models A softmax layer is still needed to predict tokens, which does not fundamentally solve the long tail problem.
NPM consists of an encoder and a reference corpus. The encoder maps the text into a fixed-size vector, and then NPM retrieves a phrase from it and fills in [MASK].
As you can see, NPM chooses the non-parametric distribution obtained on the phrases instead of using a fixed output vocabulary softmax as the output.
But training non-parametric models also brings two key problems:
1. Retrieving the complete corpus during the training process is very time-consuming and labor-intensive. Researchers have used Learning to predict phrases of arbitrary length without a decoder is difficult, and researchers solve this problem by extending span masking and phrase-level comparison goals .
In short, NPM completely removes the softmax of the output vocabulary and achieves an effective unbounded output space by predicting any number of n-grams.
The resulting model can predict "extremely rare" or even "completely unseen" words (such as Korean words), and can effectively support unlimited vocabulary sizes, which existing models cannot make it happen.
NPM Method
Pure encoder (Encoder-only) model is a very competitive representation model, but existing pure encoding models cannot make predictions with an unknown number of tokens, making their use without fine-tuning are restricted.
NPM solves this problem by retrieving a phrase to fill any number of tokens in [MASK].
InferenceThe encoder maps each distinct phrase in the reference corpus C into a dense vector space.
At test time, the encoder maps the masked query into the same vector space and fills [MASK] with phrases retrieved from C.
Here, C does not have to be the same as the training corpus and can be replaced or extended at test time without retraining the encoder.
In practice, there are a large number of phrases in the corpus, and indexing all of them is expensive.
For example, if we consider a phrase with at most l tokens (l≈20), we need to index l×|C| number of vectors, which may be time-consuming.
The researchers indexed each distinct token in C, thereby reducing the size of the index from l×|C| to |C|, and then when testing, The non-parametric distribution of all phrases is approximated by performing k-nearest neighbor searches separately for the beginning and the end.
For example, the phrase Thessaloniki composed of 4 BPE tokens is represented by the connection of c1 and c4, which correspond to the beginning (The) and the end (iki) of the phrase respectively.
Then use two vectors q_start and q_end in the same vector space to represent a query, and then use each vector to retrieve the start and end of plausible phrases before aggregating.
The premise of doing this is that the representation of the beginning and the end is good enough, that is, the starting point of q is close enough to c1, and the end point of q is close enough to c4, and this has been ensured during the training process.
Training
NPM is trained on unlabeled text data to ensure that the encoder maps the text into a good dense vector space.
There are two main problems in training NPM: 1) Complete corpus retrieval will make training very time-consuming; 2) Filling [MASK] with phrases of arbitrary length instead of tokens.
1. Masking Masking
Span masking is to mask continuous tokens whose lengths are sampled from a geometric distribution.
The researchers expanded on this:
1) If some fragments co-occur in other sequences in the batch, they are then masked to ensure that within the batch during training Positive examples (in-batch positives).
For example, the blocked clips 2010, the Seattle Seahawks, and to the all co-occur in another sequence.
But for the bigram "game," they cannot be masked together. Although they also appear in two sequences, they do not co-occur together.
2) Instead of replacing each token in the fragment with [MASK], replace the entire fragment with two special tokens [MASKs][MASKe].
For example, in the above example, regardless of the length of the masked segment, it is replaced with [MASKs][MASKe], so that the start and end vectors of each segment can be obtained, making reasoning more convenient.
2. Training target
Assuming that the masked clip is the Seattle Seahawks, during testing, the model should learn from the reference corpus The phrase the Seattle Seahawks was retrieved from other sequences of .
In the inference stage, the model obtains vectors from [MASKs] and [MASKe] and uses them to retrieve the beginning and end of the phrase from the corpus respectively.
Therefore, the training goal should encourage the vector of [MASKs] to be closer to the in the Seattle Seahawks and farther away from other tokens, and should not be the the in any phrase, such as become the in first.
We do this by training the model to approximate the full corpus to other sequences in the batch. Specifically, we train the model to retrieve the starting point of the Seattle Seahawks segment from other sequences in the same batch. and end point.
It should be noted that this mask strategy ensures that each masked span has a co-occurring segment in a batch.
From the results, NPM performs better than other baseline models under the zero-shot setting.
Among the parametric models, RoBERTa achieved the best performance, unexpectedly surpassing models including GPT-3, probably because of the pure encoder model The bidirectionality plays a crucial role, which also suggests that causal language models may not be a suitable choice for classification.
The kNN-LM method adds non-parametric components to the parametric model, and its performance is better than all other baselines. Nonetheless, relying solely on retrieval (kNN) performs poorly in GPT-2, indicating the limitations of using kNN only at inference time.
NPM SINGLE and NPM both significantly outperformed all baselines, achieving consistently superior performance on all datasets. This shows that non-parametric models are very competitive even for tasks that do not explicitly require external knowledge.
Qualitative analysis uses the prediction results of RoBERTa and NPM in sentiment analysis tasks. In the first example, cheap means not expensive, and in the second example, cheap means poor quality.
RoBERTa’s predictions for both examples were positive, while NPM made the correct prediction by retrieving contexts where cheap was used in the same context as the input Prediction.
It can also be found that the representation output by NPM can bring better word meaning disambiguation. For example, RoBERTa assigns a high similarity score between cheap and cheap.
On the other hand, NPM successfully assigns a low similarity score between cheap and cheap, which also shows that this non-parametric training with contrastive objectives is effective and can better improve representation learning, while kNN inference This kind of algorithm without training is completely impossible.
Reference: https://arxiv.org/abs/2212.01349
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