Truncating weights to 0 for Sparse lead to wrong shape fixed

[weidel-p]

Issue Number:

Objective of pull request:

When initializing Sparse with small but mixed weights, the truncating weights function of the weightutils round them to zero. As those weights are kept in the data field of the sparse matrix, the shape is wrong. This PR fixes this issue by removing these explicit zeros from the sparse matrix.

Pull request checklist

Your PR fulfills the following requirements:

• Issue created that explains the change and why it's needed
• Tests are part of the PR (for bug fixes / features)
• Docs reviewed and added / updated if needed (for bug fixes / features)
• PR conforms to Coding Conventions
• PR applys BSD 3-clause or LGPL2.1+ Licenses to all code files
• Lint (flakeheaven lint src/lava tests/) and (bandit -r src/lava/.) pass locally
• Build tests (pytest) passes locally

Pull request type

Please check your PR type:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation changes
• Other (please describe):

What is the current behavior?

What is the new behavior?

Does this introduce a breaking change?

• Yes
• No

Supplemental information