Bug Fix - IO Binning Mode. Signed by pbonazzi

[pbonazzi]

Issue Number: #235

Objective of pull request: Fix the bug of Event.fill_tensor(binning_mode="SUM") which was not performing as expected.

Pull request checklist

Your PR fulfills the following requirements:

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

Pull request type

Please check your PR type:

• [X] 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?

• Event.fill_tensor(binning_mode="SUM") was not summing events to each pixel, it was behaving the same way as the "OR" operation

What is the new behavior?

• Now, the operation correctly add multiple event per pixels when needed.

Does this introduce a breaking change?

• [ ] Yes
• [X] No

Supplemental information

The implementation uses np.add.at because torch introduced the function torch.index_add only in versions >= 2.0 and I didn't want to introduce versioning issues.

[bamsumit]

@pbonazzi looks like there are linting error, please fix them. Look at the errors here or run linting locally using flakeheaven lint src/lava tests