Since Keras API as defined as layers, how would it be used to implement the word2vec?
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Your question is really broad.. But there are many manuals and tutorials which describe how to do it step by step.
My personal favorite would be:
Practical Text Classification With Python and Keras - It has a very detailed explanation of every step of the implementation while remaining practical.
You can also try:
A Word2Vec Keras tutorial - This one focuses more on understanding how word2vec embedding works, but it also contains an implementation.
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Tensorflow released a version, however, it's missing negative sampling, which is very important in word2vec
