ONNX export of a reloaded NgramExtractingTransformer (useAllLengths: true) writes min_gram_length = ngramLength — the exported graph counts only the longest n-grams

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Valutazione

Difficoltà
3/5
Tempo stimato
1-2 giorni
Idoneità per principianti
72/100
Tipo di issue
Bug
Chiarezza
Specificata chiaramente
Stato di attività
Attiva
Stack tecnologico
csharp, machine-learning
Ambito
backend, machine-learning

Direzione di ricerca

Start by tracing ONNX export for NgramExtractingTransformer, comparing the export path immediately after Fit with the path after Model.Save and Model.Load. The issue provides a minimal C# reproduction and expected graph attributes: min_gram_length should be 1 with useAllLengths: true. Reproduce the value mismatch on the provided text output and confirm the reloaded export counts all n-gram lengths.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

area-ONNX untriaged
System information
  • Microsoft.ML 5.0.1, Microsoft.ML.OnnxConverter 0.23.1, Microsoft.ML.OnnxRuntime 1.31.0
  • .NET 10 (net10.0), macOS arm64
Describe the bug

An NgramExtractingTransformer created with useAllLengths: true exports to ONNX correctly when it is converted straight after Fit. After a Model.Save / Model.Load round trip, the same transformer exports a TfIdfVectorizer node with min_gram_length equal to max_gram_length. The resulting graph counts only the longest n-grams. The conversion reports success, so nothing shows that the graph is wrong.

To reproduce

Text column of the UCI SMS Spam Collection (4,459 training rows, 1,115 compared):

var t = ml.Transforms.Text.TokenizeIntoCharactersAsKeys("c", "text")
    .Append(ml.Transforms.Text.ProduceNgrams("text", "c", ngramLength: 4, useAllLengths: true,
        weighting: NgramExtractingEstimator.WeightingCriteria.Tf))
    .Fit(train);

The text output vector (width 75,141) from ML.NET, compared with the same output of the exported graph run in ONNX Runtime:

rows that differ
t exported directly (ml.Model.ConvertToOnnx(t, train, stream, "text")) 0 / 1,115
ml.Model.Save(t, …) → ml.Model.Load(…) → export 1,115 / 1,115

In the graph of the reloaded transformer, the TfIdfVectorizer node has min_gram_length: 4, max_gram_length: 4 while ngram_counts is [0, 112, 6500, 59291]. The pool holds 1-, 2-, 3- and 4-grams, but only 4-grams are counted, so every 1–3-gram slot is 0. A logistic regression trained on these features scores every row of the reloaded export at about its intercept (probability ≈ 0.0017; ML.NET gives 0.0000–1.0000).

Workaround: use one ProduceNgrams per length (useAllLengths: false, lengths 1..4) followed by Concatenate. This exports correctly after a reload.

Expected behavior

The exported min_gram_length is 1 when useAllLengths is true, whether or not the transformer went through Model.Save / Model.Load. In other words, the all-lengths setting survives deserialization in what the ONNX save path reads.

Why it matters

Models are usually exported from a saved file (train → save → export later). In that path the conversion succeeds and the graph silently produces different values. Only a value-by-value comparison catches it.

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C#
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