add _add_post_transform_node function to allow for custom post transf…

…orm nodes that are not supported by the ONNX spec yet

onnxmltools/convert/lightgbm/operator_converters/LightGbm.py

@@ -3,12 +3,14 @@
 import copy
 import numbers
 import numpy as np
+import onnx
 from collections import Counter
 from ...common._apply_operation import (
     apply_div, apply_reshape, apply_sub, apply_cast, apply_identity, apply_clip)
 from ...common._registration import register_converter
 from ...common.tree_ensemble import get_default_tree_classifier_attribute_pairs
 from ....proto import onnx_proto
+from onnxconverter_common.container import ModelComponentContainer
 
 
 def _translate_split_criterion(criterion):
@@ -222,6 +224,7 @@ def convert_lightgbm(scope, operator, container):
 
     # Create different attributes for classifier and
     # regressor, respectively
+    post_transform = None
     if gbm_text['objective'].startswith('binary'):
         n_classes = 1
         attrs['post_transform'] = 'LOGISTIC'
@@ -232,6 +235,13 @@ def convert_lightgbm(scope, operator, container):
         n_classes = 1  # Regressor has only one output variable
         attrs['post_transform'] = 'NONE'
         attrs['n_targets'] = n_classes
+    elif gbm_text['objective'].startswith('poisson'):
+        n_classes = 1  # Regressor has only one output variable
+        attrs['n_targets'] = n_classes
+        # 'Exp' is not a supported post_transform value in the ONNX spec yet,
+        # so we need to add an 'Exp' post transform node to the model
+        attrs['post_transform'] = 'NONE'
+        post_transform = "Exp"
     else:
         raise RuntimeError(
             "LightGBM objective should be cleaned already not '{}'.".format(
@@ -392,6 +402,22 @@ def convert_lightgbm(scope, operator, container):
             container.add_node('Identity', output_name,
                                operator.output_full_names,
                                name=scope.get_unique_operator_name('Identity'))
+        if post_transform:
+            _add_post_transform_node(container, post_transform)
+
+
+def _add_post_transform_node(container: ModelComponentContainer, op_type: str):
+    """
+    Add a post transform node to a ModelComponentContainer.
+
+    Useful for post transform functions that are not supported by the ONNX spec yet (e.g. 'Exp').
+    """
+    assert len(container.outputs) == 1, "Adding a post transform node is only possible for models with 1 output."
+    original_output_name = container.outputs[0].name
+    new_output_name = f"{op_type.lower()}_{original_output_name}"
+    post_transform_node = onnx.helper.make_node(op_type, inputs=[original_output_name], outputs=[new_output_name])
+    container.nodes.append(post_transform_node)
+    container.outputs[0].name = new_output_name
 
 
 def modify_tree_for_rule_in_set(gbm, use_float=False):