Expose index stats for debugging

[changhiskhan]

we want to expose the vector index stats to the user (in python) for debugging purposes:

1. index name, id, and file location
2. index type
3. if ivf_pq: num ivf partitions, num pq sub vectors, num pq bits
4. if ivf_pq: for each partition, the centroid vector for the partition and the number of vectors in that partition.

on the python side, the api can look like:

dataset.list_index => [Index]

dataset.get_index("name") -> Index

Index should have:

• index_type
• name
• index-id
• uri

IvfPqIndex (subclass of Index) should have:

• num ivf partitions
• num pq sub vectors
• num pq bits
• each ivf centroid
• num vectors in in partition

[Renkai]

I will work on this.

[changhiskhan]

This may also be useful for merging indices