uffy/mnist_example: classifier/main.go; 23 LoC

Found a possible issue in [uffy/mnist_example](https://www.github.com/uffy/mnist_example) at [classifier/main.go](https://github.com/uffy/mnist_example/blob/8d21eea8274f55c9c619ff951ed7c22b923a40d2/classifier/main.go#L188-L210)

Below is the message reported by the analyzer for this snippet of code. Beware that the analyzer only reports the first issue it finds, so please do not limit your consideration to the contents of the below message.

> range-loop variable m used in defer or goroutine at line 193

[Click here to see the code in its original context.](https://github.com/uffy/mnist_example/blob/8d21eea8274f55c9c619ff951ed7c22b923a40d2/classifier/main.go#L188-L210)

Click here to show the 23 line(s) of Go which triggered the analyzer.

```go
for _, m := range miniBatch {
wg.Add(1)

go func() {
defer wg.Done()
w, b := net.Backprop(m.Data, m.matrixValue())

lock.Lock()
defer lock.Unlock()
for l1, b1 := range nablaBiases {
for l2 := range b1 {
nablaBiases[l1][l2] += b[l1][l2]
}
}
for l1, w1 := range nablaWeights {
for l2, w2 := range w1 {
for l3 := range w2 {
nablaWeights[l1][l2][l3] += w[l1][l2][l3]
}
}
}
}()
}

```

Leave a reaction on this issue to contribute to the project by classifying this instance as a **Bug** :-1:, **Mitigated** :+1:, or **Desirable Behavior** :rocket:
See the descriptions of the classifications [here](https://github.com/github-vet/rangeclosure-findings#how-can-i-help) for more information.

commit ID: 8d21eea8274f55c9c619ff951ed7c22b923a40d2