Add example of using non-TFDS data loader

[mathpluscode]

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Description

As mentioned in #18 and #20, the usage of TFDS may not be obvious. While the inference example demos how to use a trained network on custom data, it remains challenging to use non-TFDS data loaders.

Following the release v0.3.2, the data iterator has been moved out of Experiment. It is now easier to use other data sets.

https://github.com/mathpluscode/ImgX-DiffSeg/blob/9c1f058b003f2e410e0efa7381c9e28ca342d4ee/imgx/run_train.py#L198-L206