InfoXLM
InfoXLM stands for “Information-theoretic procedure for Cross-Lingual Modeling” which is a cross-lingual language model proposed by Microsoft in 2020 and published in their paper: InfoXLM: An Information-Theoretic Framework for Cross-Lingual Language Model Pre-Training. The official code for this paper can be found in Microsoft’s official UniLM GitHub repository: unilm/infoxlm.
State-of-the-art cross-lingual pre-trained models are typically built using monolingual data with masked language modeling (MLM) objective such as BERT and XLM-R; along side bilingual data with Translation Language Modeling (TLM) objective such as XLM. InfoXLM combines these two objectives with a novel objective called XLCO or “Cross Lingual Contrast”.
\[\mathcal{L} = \mathcal{L}_{\text{MLM}} + \mathcal{L}_{\text{TLM}} + \mathcal{L}_{\text{XLCO}}\]Note to Reader
I think you should give the XLM post a read before going on.
We know how to obtain MLM and TLM from XLM model:
\[\mathcal{L}_{\text{MLM}} = - \log\frac{\exp\left( \theta_{T}\left( c_{1} \right)^{T}\text{.}\theta_{E}\left( x_{1} \right) \right)}{\sum_{x' \in \mathcal{V}}^{}{\exp\left( \theta_{T}\left( c_{1} \right)^{T}\text{.}\theta_{E}\left( x' \right) \right)}}\] \[\mathcal{L}_{\text{TLM}} = - \log\frac{\exp\left( \theta_{T}\left( c_{1} \right)^{T}\text{.}\theta_{E}\left( x_{1} \right) \right)}{\sum_{x' \in \mathcal{V}}^{}{\exp\left( \theta_{T}\left( c_{1} \right)^{T}\text{.}\theta_{E}\left( x' \right) \right)}}\] \[\ \ \ \ \ \ \ \ \ \ \ \ - \log\frac{\exp\left( \theta_{T}\left( c_{2} \right)^{T}\text{.}\theta_{E}\left( x_{2} \right) \right)}{\sum_{x' \in \mathcal{V}}^{}{\exp\left( \theta_{T}\left( c_{2} \right)^{T}\text{.}\theta_{E}\left( x' \right) \right)}}\]Where:
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$x_{1}, x_{2}$ are the masked tokens.
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$c_{1}, c_{2}$ are the corresponding contexts (the rest).
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$\theta_{E}$ is a look-up function that returns the token embeddings.
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$\theta_{T}$ is a Transformer that returns the final hidden vectors in position of $x_{1}$.
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$\mathcal{V}$ is the vocabulary.
XLCO
XLCO stands for “Cross-lingual Contrast” which is a new objective for pre-training cross-lingual language models inspired by the unified information-theoretic framework. The goal of XLCO is to distinguish the translation of an input sentence from a set of negative examples. The formula of this objective is:
\[\mathcal{L}_{\text{XLCO}} = - \log\frac{\exp\left( \theta_{Q}\left( c_{1} \right)^{T}\text{.}\theta_{K}\left( c_{2} \right) \right)}{\sum_{c' \in \mathcal{N}}^{}{\exp\left( \theta_{Q}\left( c_{1} \right)^{T}\text{.}\theta_{K}\left( c' \right) \right)}}\]Where:
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$\theta_{Q}$ is the query encoder that encodes $c_{1}$ and is updated by back-propagation.
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$\theta_{K}$ is the key encoder that encodes $\mathcal{N}$.
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$\mathcal{N}$ is the negative examples distribution which is organized as a queue, where a newly encoded example is added while the oldest one is popped from the queue.
The query encoder and the key encoder are initialized with the same parameters, and fill the queue with a set of encoded examples until it reaches the desired size $\left| \mathcal{N} \right|$. Notice that the size of the queue remains constant during training. In the paper, they used a queue of length equals to $131,072$.
Mixup Contrast
Mixup Contrast is an augmentation method the researcher used when applying XLCO task. It goes like this; for each parallel sentence $\left\langle c_{1},\ c_{2} \right\rangle$, they concatenated it with a randomly sampled translation pair $\left\langle d_{1},\ d_{2} \right\rangle$ from another parallel corpus. The two pairs are concatenated in a random order like $\left\langle c_{1}d_{1},\ c_{2}d_{2} \right\rangle$ or $\left\langle c_{1}d_{2},\ d_{1}c_{2} \right\rangle$. This method encourages pre-trained models to learn sentence boundaries and to distinguish the order of multilingual texts.
Results
In this paper, the researchers followed the model configurations of XLM-R when creating InfoXLM. InfoXLM~base~ used the Transformer architecture with 12 layers and 768 hidden . InfoXLM~large~ used the Transfoerm architecture with 24 layers and 1,024 hidden states.
Then, they initialized the parameters with XLM-R. The model was optimized using Adam optimizer with a batch size of $2048$ for a total of $150$K steps for InfoXLM~base~, and $200$K steps for InfoXLM~large~. The learning rate is scheduled with a linear decay with $10$K warmup steps, where the peak learning rate is set as $0.0002$ for InfoXLM~base~, and $0.0001$ for InfoXLM~large~. The momentum coefficient is set as $0.9999$ for InfoXLM~base~ and $0.999$ for InfoXLM~large~. Then, they compared InfoXLM with:
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mBERT which was pre-trained with MLM on Wikipedia in 102 languages
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XLM which was pre-trained with both MLM and TLM tasks on Wikipedia in 100 languages
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XLM-R which was pre-trained with MLM to the large CC-100 corpus in 100 languages with much more training steps.
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UNICODER which was initialized with XLM-R and they trained it using both MLM and TLM.
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InfoXLM (without XLCO).
And the following table shows this comparison on XNLI using 15 different languages. The model number #M=N indicates that each language had a different model) while #M=1 means only one model is used for all languages. Also, results with “*” are taken from this paper while “(reimpl)” means that the researchers have re-implemented it. Results of InfoXLM and XLM-R (reimpl) are averaged over five runs with different seeds:
The past results show that InfoXLM outperforms all baseline models on the two evaluation settings of XNLI. Moreover, removing XLCO object hurts the performance which shows that cross-lingual contrast is helpful for zero-shot transfer in most languages.