Add index pointer of each token within doc_freq

[eramirem]

Problem

If we want to use the output of BM25Encoder._encode_single_document to form a scipy.sparse array, we cannot use the mmh3 hashes returned as indices, but we rather need the positions of the non-null keywords within the document frequency object.

Solution

Add a flag to specify whether you want the index positions to be returned by _encode_single_document

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update
• Infrastructure change (CI configs, etc)
• Non-code change (docs, etc)
• None of the above: (explain here)

Test Plan

Describe specific steps for validating this change.