`chapter11_part04_sequence-to-sequence-learning.ipynb` never defines nor imports `TransformerEncoder`

[jchwenger]

Hi @fchollet,

Something came back to me, regarding these notebooks: chapter11_part04_sequence-to-sequence-learning.ipynb never defines or imports TransformerEncoder, as it comes a bit after the passage that is the basis for chapter11_part03_transformer.ipynb, in which the Encoder-only version is presented, and where the class is defined!

I'm assuming these have been automatically generated from the book somehow, but in case, for usability, one has to go back to part03 to get the class definition!

[ifond]

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