word synonym / antonym detection

I would suggest a following pipeline:

1. Construct a training set from existing dataset of synonyms and antonyms (taken e.g. from the Wordnet thesaurus). You’ll need to craft negative examples carefully.
2. Take a pretrained model such as BERT and fine-tune it on your tasks. If you choose BERT, it should be probably BertForNextSentencePrediction where you use your words/prhases instead of sentences, and predict 1 if they are synonyms and 0 if they are not; same for antonyms.