Implement a backward for Heaviside

Context

Since the Heaviside function is non-differentiable, the classical approach is to use a surrogate gradient for the backward pass. The most common gradient function used is the shifted arctan function :

For the forward pass, th :

S[t] = Heaviside(U[t] - U_{\rm thr}) \approx \frac{1}{π}\text{arctan}(\pi U) \\

Thus for the backward :

\frac{\partial \tilde{S}}{\partial U} = \frac{1}{(1+(U \pi)^2)}

added Feature 🚀 StatusWork in Progress TopicOperator labels

assigned to @jeromeh

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• 8053a48b...0d7ea895 - 25 commits from branch eclipse/aidge:dev
• 2ee9cdf9 - feat: Add surrogate backward function for Heaviside operator
• 178652ce - Add Heaviside backward surrogate and associated tests
• 625009b6 - Improve test for Heaviside backward
• 8a6cb8c1 - Add randomness in heaviside backward test
• 4c4e8be4 - Format file and remove unused variable
• 23ce74a5 - Improve Heavisise Backward Test

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• 2b1afb4f - Fix includes

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• 2b1afb4f...d34c4621 - 36 commits from branch eclipse/aidge:dev
• 92820583 - feat: Add surrogate backward function for Heaviside operator
• 273b75f3 - Add Heaviside backward surrogate and associated tests
• 89229b87 - Improve test for Heaviside backward
• a847cd2e - Add randomness in heaviside backward test
• f877db57 - Format file and remove unused variable
• 892d4f15 - Improve Heavisise Backward Test
• 04129dbd - Fix includes

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added 14 commits

• 04129dbd...da5f38fa - 7 commits from branch eclipse/aidge:dev
• 28ca848f - feat: Add surrogate backward function for Heaviside operator
• 2d63028a - Add Heaviside backward surrogate and associated tests
• 09bec268 - Improve test for Heaviside backward
• eff82020 - Add randomness in heaviside backward test
• c92b78e4 - Format file and remove unused variable
• 9c83962a - Improve Heavisise Backward Test
• e7c9550d - Fix includes

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@jeromeh Issues on Windows...

Should be fixed.

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• 3be71485 - Include <cmath> in HeavisideImpl kernels

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• e02f7007 - Define _USE_MATH_DEFINES in kernel header

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• 3a44ecdc - Replace cmath with math.h

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• 6db22c4a - Enable Math Constants and Use cmath Header

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• eeded572 - Add _USE_MATH_DEFINES to unit tests

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removed StatusWork in Progress label

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requested review from @cmoineau

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• 3177f599 - Only define _USE_MATH_DEFINES on Windows platforms

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