Avoid concatenation if not needed (onnx#1110)

* Avoid concatenation if not needed

Signed-off-by: Xavier Dupre <[email protected]>

* change

Signed-off-by: Xavier Dupre <[email protected]>

---------

Signed-off-by: Xavier Dupre <[email protected]>

Author: xadupre

CHANGELOGS.md

@@ -2,6 +2,8 @@
 
 ## 1.18.0
 
+* Converter for OneHotEncoder does not add a concat operator if not needed,
+  [#1110](https://github.com/onnx/sklearn-onnx/pull/1110)  
 * Function ``to_onnx`` now forces the main opset to be equal to the
   value speficied by the user (parameter ``target_opset``),
   [#1109](https://github.com/onnx/sklearn-onnx/pull/1109)  

skl2onnx/operator_converters/one_hot_encoder.py

@@ -238,8 +238,11 @@ def convert_sklearn_one_hot_encoder(
         result.append(ohe_output)
         categories_len += len(categories)
 
-    concat_result_name = scope.get_unique_variable_name("concat_result")
-    apply_concat(scope, result, concat_result_name, container, axis=-1)
+    if len(result) == 1:
+        concat_result_name = result[0]
+    else:
+        concat_result_name = scope.get_unique_variable_name("concat_result")
+        apply_concat(scope, result, concat_result_name, container, axis=-1)
 
     reshape_input = concat_result_name
     if np.issubdtype(ohe_op.dtype, np.signedinteger):