[Python][ML] Fix and re-enable rdataloader xgboost tutorial

[siliataider]

This Pull request:

Changes or fixes:

This PR fixes the ml_dataloader XGBoost tutorial, which had to be turned off because it crashed sporadically in CI, 2 separate issues are fixed here:

• The previous way of splitting cluster boundaries for train/val counts was splitting every cluster independently using the validation split ratio, and counting on the last cluster to compensate for whatever rounding shortfall had built up from the earlier ones, if the last cluster didn't have enough entries, we sometimes ended up with less training entries than the actual batch size we promise, making us read past the rows that were actually written.
• Separately, the "weights" column was handled differently: it was being handed to Python as a NumPy view into the C++ data, while the "features" and "target" columns were being handed over as copies. The view only stayed valid as long as the object that produced it was still alive, but that object didn't actually live long enough in this particular tutorial, as the generator we create goes out of scope at the end of the load_data function, so the weights view could end up pointing at memory that was already gone.
  • To fix this we made everything work the same way: features, target, and weights are now all views, and none of them depend on that short-lived object anymore. The data itself is moved out into its own independent object, so it survives for as long as the view into it is being used.
  • While working on this we made an interface change: columns previously required all training columns + targets + wieghts to be present in that list, it is no longer the case

[vepadulano]

Nice work investigating this! The changes look good overall but I left some minor comments related to commit and code structure

[vepadulano]

Thank you!

[github-actions]

Test Results

    22 files      22 suites   3d 15h 28m 59s ⏱️
 3 857 tests  3 856 ✅ 0 💤 1 ❌
75 739 runs  75 735 ✅ 2 💤 2 ❌

For more details on these failures, see this check.

Results for commit aba9e2f.