add PReLU

[LehaoLin]

I am not familiar with circom code.
But I try to write a PReLU.circom, PReLU_test.circom, PReLU.js
I found that PReLU is quite similar to Leaky_relu, but the main difference is that in PReLU, alpha is not a fixed parameter but a learnable parameter, typically initialized randomly and then adjusted through backpropagation during training.

However, I try to set the alpha in the block of the circom template but get error

error[T3001]: Non quadratic constraints are not allowed!
   ┌─ "/Users/linlehao/project/circomlib-ml/circuits/PReLU.circom":17:5
   │
17 │     neg <== isNegative.out * alpha * in;
   │     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ found here

Therefore, I use the same code with LeakyReLU as

// PReLU layer
template PReLU (alpha) {  // alpha is 10 times the actual alpha, since usual alpha is 0.2, 0.3, etc.
    signal input in;
    signal input out;
    //signal input alpha; 
    signal input remainder;
    
    component isNegative = IsNegative();

    isNegative.in <== in;

    signal neg;
    neg <== isNegative.out * alpha * in;

    out * 10 + remainder === neg + 10 * in * (1 - isNegative.out);
}

Hope you can help it more.