Score Fix for Binary Quantized Vector and Setting Default value in case of shard level rescoring is disabled for oversampling factor

[Vikasht34]

Description

Change1

Problem
In Disk Vector Mode, while calculating the score for a binary quantized vector, the system was incorrectly using Inner Product as the similarity function. However, for binary quantized vectors, the correct distance metric should be Hamming Distance.

Impact:
This incorrect scoring mechanism led to poor results, particularly in cases with multiple segments. During the process of reducing to the top k candidates, the best candidates (those with the most relevant results) were being filtered out. As a result, the recall was zero, meaning no relevant results were being returned, which critically affected the accuracy of search results.

Expected Behavior:
The system should use Hamming Distance for binary quantized vectors, ensuring that the correct similarity metric is applied. This would allow the best candidates to be retained during the top-k reduction process, thereby maintaining high recall and improving overall search performance in multi-segment scenarios.

Change 2:

Problem:
The system already had a setting to disable shard-level rescoring, allowing for rescoring at the segment level. However, with a recent PR, a default oversampling factor was reintroduced to be applied when the user did not explicitly provide one. Unfortunately, this default oversampling factor was being applied in both cases — regardless of whether shard-level rescoring was enabled or disabled. With Segment level , we don't need high oversampling factor.

Solution:
This PR introduces changes to ensure that the default oversampling factor is only applied when shard-level rescoring is disabled. When segment-level rescoring is enabled, the system overrides the default oversampling factor, ensuring the correct sampling factor is applied based on dimesion.

Related Issues

Resolves #[Issue number to be closed when this PR is merged]

Check List

• New functionality includes testing.
• New functionality has been documented.
• API changes companion pull request created.
• Commits are signed per the DCO using --signoff.
• Public documentation issue/PR created.

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