Skl2onnx wrapper, deleting redundant converter code (#222)

* including dependency on skl2onnx for onnxmltools 1.4
* creating wrapper for scikitlearn to onnx model conversion
* deleting sklearn converter files and tests to remove redundant code

Author: vinitra

onnxmltools/convert/common/utils.py

@@ -20,6 +20,15 @@ def sklearn_installed():
     except ImportError:
         return False
 
+def skl2onnx_installed():
+    """
+    Checks that *skl2onnx* converter is available.
+    """
+    try:
+        import skl2onnx
+        return True
+    except ImportError:
+        return False
 
 def coreml_installed():
     """

onnxmltools/convert/main.py

@@ -7,7 +7,6 @@
 from ..proto import onnx
 from .common import utils
 
-
 def convert_coreml(model, name=None, initial_types=None, doc_string='', target_opset=None,
                    targeted_onnx=onnx.__version__ , custom_conversion_functions=None, custom_shape_calculators=None):
     if not utils.coreml_installed():
@@ -55,6 +54,9 @@ def convert_sklearn(model, name=None, initial_types=None, doc_string='', target_
     if not utils.sklearn_installed():
         raise RuntimeError('scikit-learn is not installed. Please install scikit-learn to use this feature.')
 
-    from .sklearn.convert import convert
-    return convert(model, name, initial_types, doc_string, target_opset, targeted_onnx,
+    if not utils.skl2onnx_installed():
+        raise RuntimeError('skl2onnx is not installed. Please install skl2onnx to use this feature.')
+
+    from skl2onnx.convert import convert_sklearn as convert_skl2onnx
+    return convert_skl2onnx(model, name, initial_types, doc_string, target_opset,
                    custom_conversion_functions, custom_shape_calculators)