index factory bugfix

[SylviaZiyuZhang]

• Does this PR have a descriptive title that could go in our release notes? Yes and no, it is a mini bug fix.
• Does this PR add any new dependencies? No.
• Does this PR modify any existing APIs? No.
  • Is the change to the API backwards compatible? N/A
• Should this result in any changes to our documentation, either updating existing docs or adding new ones? No.

Reference Issues/PRs

Fixes #649

What does this implement/fix? Briefly explain your changes.

If an index is built with the create_instance function in the index factory, assert(_graph_store->get_total_points() == _max_points + _num_frozen_pts); at the end of search_for_point_and_prune fails during the link() stage. For example. Minimal reproduction:

#include "abstract_index.h"
#include "index.h"
#include "index_config.h"
#include "index_factory.h"
#include "ann_exception.h"
#include "utils.h"

diskann::IndexWriteParameters index_build_params =
    diskann::IndexWriteParametersBuilder(L, R)
        .with_filter_list_size(Lf)
        .with_alpha(alpha)
        .with_saturate_graph(false)
        .with_num_threads(num_threads)
        .build();

diskann::IndexConfig config = diskann::IndexConfigBuilder()
    .with_metric(diskann::Metric::COSINE)
    .with_dimension(embedding_dim)
    .with_max_points(500)
    .is_dynamic_index(true)
    .with_index_write_params(index_build_params)
    .is_enable_tags(true)
    .is_use_opq(true)
    .is_pq_dist_build(false)
    .with_data_type("float")
    .build();
diskann::IndexFactory index_factory(config);
diskann_index = index_factory.create_instance();

_graph_store->get_total_points() would be 502 instead of 501 (with 1 frozen point by default).

Any other comments?

[SylviaZiyuZhang]

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