Multiclass margin classifier demo has a very long execution time about qml

[glassnotes]

@antalszava has updated this one to tape mode. The new version takes between 7-8 minutes on my machine, I expect it will build for the demo page in ~10 minutes which is much better than before!

[mariaschuld]

We could simply use 50 instead of 100 optimisation steps, which should make things twice as fast. The final plot is quite noisy anyways!

@thisac are you still upgrading this demo to tape mode, so that you could change total_iterations = 100 to total_iterations = 50? Or shall I make a PR?

[josh146]

The new backprop default might make a big difference without any changes, even?

[glassnotes]

The build in tape mode confirms the demo time is now down to ~7.5 minutes. It may be possible to improve this further, but will close this now since this is a significant improvement and the build time is now below the recommended threshold.