Bugfix: In IndexUpdater, cast pids to Int before saving

[AshwinParanjape]

I get an error while using IndexUpdater. This PR fixes it.

Here's the minimal example script

import os
from pathlib import Path

from colbert import Indexer, Searcher, IndexUpdater
from colbert.infra import ColBERTConfig, Run, RunConfig

old_books = [
    "The Adventures of Sherlock Holmes",
    "The Adventures of Tom Sawyer",
    "The Adventures of Huckleberry Finn",
]

modern_books = [
    "The song of ice and fire"*10,
    "Harry Potter and the Sorcerer's Stone"*10,
    "The Hunger Games"*10,
    "The Fellowship of the Ring"*10,
    "The Two Towers",
    "The Return of the King",
    "The Hobbit",
    "The Lord of the Rings",
    "The Hitchhiker's Guide to the Galaxy",
    "Seventy-Seven Clocks",
    "I, Robot",
    "Life, the Universe and Everything",
]

new_index = "tiny_index"

# Download the colbert model to this path
colbert_checkpoint = "colbertv2.0/"
if __name__ == "__main__":
    with Run().context(
        RunConfig(
            nranks=1,
            root=new_index,
            experiment="",
        )
    ):
        config = ColBERTConfig(nbits=2, root=new_index, doc_maxlen=100)
        indexer = Indexer(checkpoint=colbert_checkpoint, config=config)

        indexer.index(
            name="msmarco.nbits=2",
            collection=modern_books
        )

        searcher = Searcher(index="msmarco.nbits=2", config=config)
        config = ColBERTConfig(nbits=2, root=new_index, doc_maxlen=100)
        index_updater = IndexUpdater(
            config=config, searcher=searcher, checkpoint=colbert_checkpoint
        )

        index_updater.add(old_books)
        index_updater.persist_to_disk()
        results = searcher.search("Sherlock", 3)
        print(results)

Leads to the following error

Traceback (most recent call last):
  File "test_incremental_build_minimal.py", line 79, in <module>
    results = searcher.search("Sherlock", 3)
  File ".../ColBERT/colbert/searcher.py", line 61, in search
    return self.dense_search(Q, k, filter_fn=filter_fn)
  File ".../ColBERT/colbert/searcher.py", line 108, in dense_search
    pids, scores = self.ranker.rank(self.config, Q, filter_fn=filter_fn)
  File ".../ColBERT/colbert/search/index_storage.py", line 84, in rank
    scores, pids = self.score_pids(config, Q, pids, centroid_scores)
  File ".../ColBERT/colbert/search/index_storage.py", line 142, in score_pids
    pids = IndexScorer.filter_pids(
RuntimeError: expected scalar type Int but found Long

This PR fixes this issue, resulting in the following output

([10, 6, 13], [1, 2, 3], [9.62167739868164, 8.728744506835938, 8.65077018737793])

[sarisel]

I think this error only happens when running on CPU.

[AshwinParanjape]

That may be true, I was testing it on a CPU machine