demo/bug: fixed_train_size_demo

[sameerashahh]

Describe the bug
DNN.init() takes 3 positional arguments but 4 were given

To Reproduce
Steps to reproduce the behavior:

1. Go to demos/challenges/fixed_train_size_demo.ipynb
2. Run demo cell by cell
3. Error occurs in the 3rd cell.

Expected behavior
The cell should run so data can be generated.

[carynbear]

Assigned to Vincent