Building ivf for large datasets

[jenhsia]

When using function _build_ivf(self) for a large corpus, it often gets stuck at the codes = codes.sort() step.
To avoid sorting of a massive list, we can:

1. Create an ivf_dict which maps from partition index to the list of embedding indices that belong to that partition.
2. Using ivf_dict, we can easily create the following without soring:

• a sorted list of embedding indices (ivf) by just concatenating the dictionary values, and
• a list of the number of embeddings belonging to each partition (ivf_lengths).