fix: save index during creation for hnswlib - #1424

This PR is related to issue #1351

The issue is that using two time the same work_dir lead to error.

from docarray.documents import ImageDoc
from docarray.index import HnswDocumentIndex

# create a Document Index
index = HnswDocumentIndex[ImageDoc](work_dir='/tmp/test_index2')
index = HnswDocumentIndex[ImageDoc](work_dir='/tmp/test_index2') # second time is failing

The error is that hnswlib index file .bin is created in function index() but not _create_index(). Second time it tries to load .bin file from the work_dir, which doesn't exist.