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Trying to understand the dimensions of the Multihead Attention component in Transformer referring the following tutorial https://www.tensorflow.org/tutorials/text/transformer#setup

There are 2 unknown dimensions - depth and d_model which I dont understand.

For example, if I fix the dimensions of the Q,K,V as 64 and the number_of_attention_heads as 8, and input_embedding as 512 , can anyone please explain what is depth and d_model?

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  • d_model is the dimensionality of the representations used as input to the multi-head attention, which is the same as the dimensionality of the output. In the case of normal transformers, d_model is the same size as the embedding size (i.e. 512). This naming convention comes from the original Transformer paper.

  • depth is d_model divided by the number of attention heads (i.e. 512 / 8 = 64). This is the dimensionality used for the individual attention heads. In the tutorial you linked, you can find this as self.depth = d_model // self.num_heads. Each attention head projects the original representation into a smaller representation of size depth, then computes the attention, and then all the attention head results are concatenated together, so that the final dimensionality is again d_model. You can find more details on the individual computations in this other answer.

Note that the implementation of the multi-head attention in the tutorial is not a straightforward implementation from the original paper but it is equivalent: in the original paper, there are different matrices $W_i^Q, W_i^K, W_i^V$ for each attention head $i$, while in the implementation of the tutorial there are combined matrices $W^Q, W^K, W^V$ that compute the projection for all attention heads, which is then split into the separate heads by means of the function split_heads.

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  • $\begingroup$ Excellent. This is so helpful $\endgroup$ Apr 30 '21 at 19:22
  • $\begingroup$ Just to confirm, then the shape of Wq = Wk = Wv = (word_embedding_dim,depth) which is (512,64) in this case? $\endgroup$ Apr 30 '21 at 19:31
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    $\begingroup$ If there were separate $W_i^Q$, $W_i^K$ and $W_i^V$ for each head, then yes. However, in the tutorial they have combined project projections, so their shapes are $d_{model} \times d_{model}$, and after multiplying the input, the outputs are sliced into smaller matrices with shape $d_{model} \times depth$. $\endgroup$
    – noe
    Apr 30 '21 at 19:55
  • $\begingroup$ Thanks. I got it now. $\endgroup$ Apr 30 '21 at 23:29

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