[AutoML API Samples] Move Datasets used by each sample to its own sample's folder

@cartacioS @daholste (For next week, after //BUILD/, not urgent. To be done with a PR against MASTER).

Since most datasets used by the samples are small, we're selecting the approach of having each dataset placed within each sample's folder, so each sample is more "autonomous".
Sure, sometimes there might be redundancy, but some samples might choose to change the dataset, headers, etc. so it will not impact other samples.

Current AutoML samples have the datasets in a central folder in the root.

Please, when possible let's step out of that approach and move each dataset to its own sample.