Improve randomness of stub encoder

[acatav]

Problem

For low dimension we encounter very close vectors from our stub encoder.

Solution

Improve randomness of stub encoder output by setting each dimension value sign randomly.
This fix reduce the likelihood for collisions by 2 ** -i

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

the change is part of the tests infra