[mlpack-git] [mlpack] Fix drop out layer (#463)

[Marcus Edel]

> Sorry for my misunderstanding, I should open an issue before I treat it as a bug next time.

No worries, I guess the pull request makes it sometimes easier to talk about changes.

> Do you mean you want to set the ration when you call the Forward function?

I would put ``scale = 1.0 / (1.0 - ratio);``at the beginning of the ``Forward(...)`` function, so the ``scale`` value is calculated even in prediction only mode. Additionally a user could change the scale value afterwards through the ``Ratio()`` function.

```
template<typename eT>
void Forward(const arma::Mat<eT>& input, arma::Mat<eT>& output)
 {	
    scale = 1.0 / (1.0 - ratio);

    // The dropout mask will not be multiplied in the deterministic mode
    // (during testing).
    if (deterministic)
    {
      output = input;

      if (rescale)
        output *= scale;
    }
    else
    {
      // Scale with input / (1 - ratio) and set values to zero with probability
      // ratio.
      mask = arma::randu<arma::Mat<eT> >(input.n_rows, input.n_cols);
      mask.transform( [&](double val) { return (val > ratio); } );
      output = input % mask * scale;
    }
}
```

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https://github.com/mlpack/mlpack/pull/463#issuecomment-150184035
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