Explanation of build_vocab in torch and it's association with pre-trained embeddings
Can some explain me what is build_vocab in torch, it is not clear from online documentation? Why do we need it and it's relation to pre-trained embeddings? 

I think you confuse pytorch and torchtext here. In torchtext (a package that provides processing utilities and popular datasets for natural language) you can run build_vocab of a Field to iterate over your dataset in order to build up the vocabulary.

Take also a look here:
https://torchtext.readthedocs.io/en/latest/data.html#torchtext.data.Field.build_vocab
