Fix L1 regularization in case where weight is zero

[akropp]

In the case where there was a 0 in the weights, the L1Regularizer would produce a nonfinite value in the gradient due to a divide-by-zero. This code was just attempting to implement the "sign" function, so replaced it with that.

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[rcurtin]

Thanks!! This is a nice catch. 👍 If you want to update HISTORY.md to point out the fix, feel free, otherwise I'll push a commit before merge.

[mlpack-bot]

Second approval provided automatically after 24 hours. 👍

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