Embedding generation is slow due to inefficient search

[Bhuvan-21]

In the emb_generate.py file, for every index, the program checks whether it is in the existing list of indices here

if idx in exisiting_indices:
    continue

This operation is O(n) where n is the length of existing indices, this can be easily avoided by converting this list into a set before starting the loop. This is a minor change, hopefully the authors can take care of this in the next commit.

exisiting_indices = set(exisiting_indices)