Hello!

I am trying to use Flux to train a neural network but, in this case I don’t have the target values for the output. I will try to formulate a simpler version of my problem.

Suppose I have an input

```
input = [0.8 0.1 0.3]
```

and, using the parameters initialized by Flux, I obtain the following output:

```
output = [0.3 0.8 0.3]
```

As I said, I don’t have a real output (target) value to compare to the output obtained by Flux, in order to update the weights and to produce a better output. However, I know that those output values will be used as an input for a different problem (in my real case, it is an optimization problem). So, assume that the output values will be used as inputs for the following function:

```
fc = 3y_1 + 2y_2 + 5y_3
```

Adopting the values from `output`

in the `fc`

expression, we obtain a result of 4. However, I know the optimum value for this function, say it is 2.5.

How can implement a neural network in Flux such that this difference (in the example, `4 - 2.5 = 1.5`

) is always used to update the weights and to obtain a new value for the NN prediction?