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

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• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation changes
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What is the current behavior?

What is the new behavior?

Does this introduce a breaking change?

• Yes
• No

Supplemental information

[bamsumit]

@ccaccavella thanks a lot for identifying this and issuing a fix :)