[RDF] Add internal GetDatasetGlobalClusterBoundaries utility to retrieve cluster entry ranges

[siliataider]

This Pull request:

Adds GetDatasetGlobalClusterBoundaries as an internal utility to retrieve entry ranges for each cluster in a TTree or RNTuple based RDataFrame.

When possible, the files are processed in parallel with a ROOT::TThreadExecutor.

It returns a list of cluster boundaries across files, using a global offset.

This utility is required by the RDataLoader to shuffle and prefetch data for ML training.

Changes

• RNTupleDS: add GetDatasetGlobalClusterBoundaries as a friend function to access private members fNTupleName and fFileNames
• RNTupleDS: set fNTupleName in the single file constructor (like the multi file constructor)
• RInterface: add GetDatasetGlobalClusterBoundaries() implementation for both TTree and RNTuple datasources (dispatches to existing GetClustersAndEntries implementations)

[siliataider]

@vepadulano the multithreaded approach may not be so straightforward: the global offset depends on the cumulative entry count from previous files, opened sequentially

We could maybe think of a two step approach, collecting clusters in parallel and then sequentially adjusting with a global offset somehow preserving the original order of the files..

[github-actions]

Test Results

    22 files      22 suites   3d 6h 37m 20s ⏱️
 3 830 tests  3 818 ✅  1 💤 11 ❌
75 611 runs  75 582 ✅ 18 💤 11 ❌

For more details on these failures, see this check.

Results for commit 5442dae.

♻️ This comment has been updated with latest results.

[vepadulano]

I think a better function naming would be GetClusterBoundaries. More in general, I think we should take this chance to merge the functionalities of ROOT::Internal::TreeUtils::GetClustersAndEntries and ROOT::Internal::RDF::GetClustersAndEntries.

All in all, let's discuss how much extra work would be to implement GetClustersAndEntries as one function in the Internal::RDF namespace that dispatches the cluster boundary + number of entries retrieval depending on whether the input dataset is TTree or RNTuple

[siliataider]

@vepadulano as a followup to the conversation we had earlier, I rewrote GetClusterBoundaries as a function that detects the type of the RDataSource we have, and dispatches accordingly either to ROOT::Internal::TreeUtils::GetClustersAndEntries for TTree or ROOT::Internal::RDF::GetClustersAndEntries for RNTuple, then unifies the results into a common return format.
It also now uses a pool of threads to open the files and get the metadata concurrently making it a bit faster.

[vepadulano]

Very nice! I left some comments to refine the PR.

[vepadulano]

Nice work, thanks!