Sequential version of quadratic problem in TensorFlow

[fsschneider]

The quadratic_deep problem for PyTorch has been updated and slightly changed. It is now re-written as a sequential "neural network". This allows compatibility for example with BackPACK.

The TensorFlow version should be updated accordingly. The update most likely introduces only constant or scaling changes. But to be precise, the TensorFlow version should be as equivalant as possible.

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To Do

We will do the following steps for version 1.2.0:

• [ ] Update the TensorFlow version of quadratic_deep to match our PyTorch version.