Remove the redundant loss_val

[samet-akcay]

Description

• Currently the loss in STFPM is computed as follows : loss = self.loss_val + self.loss(teacher_features, student_features). However, self.loss_val is always zero and adds no value. This PR therefore removes it from the computation.

• Fixes STFPM loss #421

Changes

• 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

Checklist

• My code follows the pre-commit style and check guidelines of this project.
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing tests pass locally with my changes