FR: Introduce Tests

[stroblme]

Is your feature request related to a problem? Please describe.
N/A

Describe the solution you'd like
Introduce tests that allow to continuously check that the following things (to be extended) are still working:

• Loss decreases within a couple of epochs
• Optimizer API is working (i.e. iterate all optimizer and check for errors)
• Data has expected shape (Batch Size x Channels x H x W) and range (0..1)
• Reproducability works (consistent data, consistent training)

Describe alternatives you've considered
N/A

Additional context
N/A