Add Alpha and Beta parameters in Backward method

Context

Solve the issue !272 for CUDA backend.

Modified files

The CUDA kernel for the following operators is modified: ILayerNorm, ShiftGELU, ShiftMax.

Detailed major modifications

Add Alpha=1 and Beta=1 parameters to Backward() method so as to ADD a contribution to the upstream gradient tensor.

mentioned in issue aidge#272 (closed)

added Fix 🔥🔥 Important ❗ labels

changed milestone to %aidge v0.6.0

approved this merge request

added 2 commits

• a7b68379 - 1 commit from branch eclipse/aidge:dev
• 301a4d64 - Add Alpha and Beta in Backward method

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merged