missing Java imports in DenseNetBuilder and DenseNetModel

#### Issue Description

I was following the quickstart tutorial, with the following basic 3 commands

```
git clone https://github.com/eclipse/deeplearning4j-examples.git
cd dl4j-examples/
mvn clean install
```

On Tuesday (13.10.2020) these steps worked successfully (on a Xubuntu 20.4 virtual machine, running openjdk 8), but I then had to switch to another task. Today (16.10.2020) I went back to the tutorial on another Xubuntu VM (running openjdk 11 this time) and the **mvn clean install** step failed with a missing symbol on several classes (List, ArrayList and Arrays), plus a **String[] is not a functional interface**, all in DenseNetBuilder and DenseNetModel classes (both in org.deeplearning4j.examples.advanced.modelling.densenet.model package).

#### Version Information

* Deeplearning4j 1.0.0-beta7
* Xubuntu 20.4, openjdk 8 and 11

#### Contributing

add the missing imports

```
import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;
```
and change
```
toArray(String[]::new)
```
into something like (my java 8 is rusty )
```
stream().toArray(String[]::new)
```