How do Transformer models handle out-of-vocabulary (OOV) words during translation?

Transformer models handle out-of-vocabulary (OOV) words, which are words not present in the model's vocabulary, through several techniques. The most basic approach is to replace OOV words with a special token, typically denoted as "UNK" (unknown). During training, the model learns a representation for the UNK token, which is used for all OOV words. This allows the model to still process sentences containing OOV words, but the model does not have any specific information about the meaning or context of these words. A more advanced technique is to use subword tokenization algorithms, such as Byte-Pair Encoding (BPE) or WordPiece. These algorithms break down words into smaller subword units, which are then used as the vocabulary. This reduces the number of OOV words, as rare words can be represented as combinations of more frequent subwords. For example, the word "unbelievable" might be broken down into "un", "believe", and "able". This allows the model to handle rare words more effectively, as it can leverage the knowledge it has learned about the individual subwords. Another technique involves using character-level embeddings. Instead of representing words as single tokens, the model represents words as sequences of characters. This allows the model to handle OOV words by learning representations for individual characters and combining them to represent the OOV word. This approach can be particularly effective for languages with rich morphology, where many words can be formed by combining prefixes, suffixes, and roots. Finally, back-translation can also help with handling OOV words. This technique involves translating the source sentence into another language and then translating it back into the source language. If the back-translated sentence contains different words than the original sentence, these new words can be added to the vocabulary or used to generate additional training data. These techniques allow Transformer models to effectively handle OOV words and to maintain good translation quality, even when the input text contains words that were not seen during training.