fixing sign of surrogate gradient for graded spikes

[ccaccavella]

Issue Number: There is no issue open. The change has been discussed with Sumit Shrestha.

Objective of pull request: The changes has been made because the implementation of the surrogate gradient in the case of a graded spike activity was wrong. Specifically, the the sign inside the exponential function was reversed.

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