Support LGBMRanker conversion (updated)

onnxmltools/convert/lightgbm/_parse.py

@@ -11,7 +11,7 @@
     Int64Type,
 )
 
-from lightgbm import LGBMClassifier, LGBMRegressor
+from lightgbm import LGBMClassifier, LGBMRegressor, LGBMRanker
 
 lightgbm_classifier_list = [LGBMClassifier]
 
@@ -21,6 +21,7 @@
 lightgbm_operator_name_map = {
     LGBMClassifier: "LgbmClassifier",
     LGBMRegressor: "LgbmRegressor",
+    LGBMRanker: "LgbmRanker",
 }
 
 
@@ -35,6 +36,8 @@ def __init__(self, booster):
         elif self.objective_.startswith("multiclass"):
             self.operator_name = "LgbmClassifier"
             self.classes_ = self._generate_classes(booster)
+        elif self.objective_.startswith("lambdarank"):
+            self.operator_name = "LgbmRanker"
         elif self.objective_.startswith(
             ("regression", "poisson", "gamma", "quantile", "huber", "tweedie")
         ):

onnxmltools/convert/lightgbm/operator_converters/LightGbm.py

@@ -566,6 +566,10 @@ def convert_lightgbm(scope, operator, container):
         # so we need to add an 'Exp' post transform node to the model
         attrs["post_transform"] = "NONE"
         post_transform = "Exp"
+    elif gbm_text["objective"].startswith("lambdarank"):
+        n_classes = 1  # Ranker has only one output variable
+        attrs["post_transform"] = "NONE"
+        attrs["n_targets"] = n_classes
     else:
         raise RuntimeError(
             "LightGBM objective should be cleaned already not '{}'.".format(
@@ -1026,3 +1030,4 @@ def convert_lgbm_zipmap(scope, operator, container):
 register_converter("LgbmClassifier", convert_lightgbm)
 register_converter("LgbmRegressor", convert_lightgbm)
 register_converter("LgbmZipMap", convert_lgbm_zipmap)
+register_converter("LgbmRanker", convert_lightgbm)

onnxmltools/convert/lightgbm/shape_calculators/Ranker.py

@@ -0,0 +1,6 @@
+# SPDX-License-Identifier: Apache-2.0
+
+from ...common._registration import register_shape_calculator
+from ...common.shape_calculator import calculate_linear_regressor_output_shapes
+
+register_shape_calculator("LgbmRanker", calculate_linear_regressor_output_shapes)

onnxmltools/convert/lightgbm/shape_calculators/__init__.py

@@ -3,3 +3,4 @@
 # To register shape calculators for lightgbm operators, import associated modules here.
 from . import Classifier
 from . import Regressor
+from . import Ranker