[ML] De-template the RDataLoader engine to cut the JIT cost

[siliataider]

This Pull request:

Changes or fixes:

De-template the RDataLoader by adding a custom RDataFrame action to load the entries as the RDataLoader needs them.

Benchmarking results

7256529 rows x 11 cols, batch 262144, 10 epochs, baseline RSS 0.26 GiB, run on a local machine

• uproot-based loader, 1 thread
• RDataLoader eager, 1 thread

	RDataLoader eager before #23473	RDataLoader eager after #23473 (master)	RDataLoader eager in this PR
setup	0.54	0.43	0.86
ep1	9.02	7.49	4.08
ep2	0.22	0.15	0.14
ep3	0.22	0.04	0.04
ep4	0.22	0.04	0.04
ep5	0.22	0.04	0.04
ep6	0.22	0.04	0.04
ep7	0.22	0.04	0.04
ep8	0.22	0.04	0.04
ep9	0.23	0.04	0.04
ep10	0.22	0.04	0.04
total	11.56	8.38	5.41
peak RSS	2.69G	1.78G	1.65G

Flow

%%{init: {"flowchart": {"nodeSpacing": 20, "rankSpacing": 30}, "themeVariables": {"fontSize": "13px"}}}%%
flowchart LR
    PY["RDataLoader(...)"] --> ENG
    PY -.->|"before (master)"| OLD["template string<br/>→ cling jits per schema"]

    subgraph ML["[ML] compiled engine + loaders"]
        ENG["engine ctor:<br/>column layout once"] --> LOAD["cluster / dataset loader<br/>(cols, vecSizes, vecPadding)"]
    end

    subgraph RDF["[RDF] type-erased custom loader action"]
        CALL["LoadCustomValues"] --> HELP["once per column:<br/>T → AppendAs⟨T⟩<br/>RVec⟨T⟩ → AppendVectorAs⟨T⟩"]
        HELP --> EXEC["each entry: void* per column<br/>→ handler → floats<br/>(vectors truncated / padded)"]
    end

    LOAD --> CALL
    EXEC --> COPY["copy into RFlat2DMatrix<br/>→ batches"]

    classDef old stroke-dasharray:5 5
    class OLD old

Loading

[github-actions]

Test Results

    24 files      24 suites   4d 2h 43m 24s ⏱️
 3 889 tests  3 888 ✅ 0 💤 1 ❌
83 245 runs  83 244 ✅ 0 💤 1 ❌

For more details on these failures, see this check.

Results for commit 05a7e89.

♻️ This comment has been updated with latest results.

[vepadulano]

Thank you! Here are some comments from my side.

[vepadulano]

Thank you, great work! I don't have further comments on the changes. Let's see it validated with a couple of CI runs.