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While reviewing the Transformer architecture, I realized something I didn't expect, which is that :

  • the positional encoding is summed to the word embeddings
  • rather than concatenated to it.

positional encoding summed to word embedding

http://jalammar.github.io/images/t/transformer_positional_encoding_example.png

Based on the graphs I have seen wrt what the encoding looks like, that means that :

  • the first few bits of the embedding are completely unusable by the network because the position encoding will distort them a lot,
  • while there is also a large amount of positions in the embedding that are only slightly affected by the positional encoding (when you move further towards the end).

graph shows positional encoding affects firsts logits a lot, last logits hardly not

https://www.tensorflow.org/beta/tutorials/text/transformer_files/output_1kLCla68EloE_1.png

So, why not instead have smaller word embeddings (reduce memory usage) and a smaller positional encoding retaining only the most important bits of the encoding, and instead of summing the positional encoding of words keep it concatenated to word embeddings?

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