title: coremltools.converters.sklearn.convert causes an error: NameError: name '_tree' is not defined
body: <p>TLDR: I can't convert my linear regression model into a model I can save like below:</p>
  <pre><code>model = coremltools.converters.sklearn.convert(regr, input_features, output_feature)
  model.save(&quot;Advertising.mlmodel&quot;)
  </code></pre>
  <p>I was working on a Raywenderlich tutorial <a href="https://www.raywenderlich.com/174-beginning-machine-learning-with-scikit-learn#toc-anchor-005" rel="nofollow noreferrer">Beginning Machine Learning with SciKit Learn</a> and I stumbled upon an error at the end of Jupyter Notebook when I convert my linear regression to a model I can save, it gives me the following error.</p>
  <pre><code>---------------------------------------------------------------------------
  NameError                                 Traceback (most recent call last)
  &lt;ipython-input-82-da16b7baefa4&gt; in &lt;module&gt;
       12 # tree.export_graphviz(model)
       13 
  ---&gt; 14 coreml_model = coremltools.converters.sklearn.convert(model, inputs, output)
       15 coreml_model.save('Advertising.mlmodel')
  
  /usr/local/lib/python3.8/site-packages/coremltools/converters/sklearn/_converter.py in convert(sk_obj, input_features, output_feature_names)
      146     # several issues with the ordering of the classes are worked out.  For now,
      147     # to use custom class labels, directly import the internal function below.
  --&gt; 148     from ._converter_internal import _convert_sklearn_model
      149 
      150     spec = _convert_sklearn_model(
  
  /usr/local/lib/python3.8/site-packages/coremltools/converters/sklearn/_converter_internal.py in &lt;module&gt;
       34 from . import _LinearSVR
       35 from . import _linear_regression
  ---&gt; 36 from . import _decision_tree_classifier
       37 from . import _decision_tree_regressor
       38 from . import _gradient_boosting_classifier
  
  /usr/local/lib/python3.8/site-packages/coremltools/converters/sklearn/_decision_tree_classifier.py in &lt;module&gt;
       14 
       15 model_type = &quot;classifier&quot;
  ---&gt; 16 sklearn_class = _tree.DecisionTreeClassifier
       17 
       18 
  
  NameError: name '_tree' is not defined
  </code></pre>
  <p>It's weird because according to Apple's official documentation at <a href="https://apple.github.io/coremltools/generated/coremltools.converters.sklearn.convert.html" rel="nofollow noreferrer">github.io/coremltools</a>, their implementation is the same as Raywenderlich and still doesnt work for me.</p>
  <p>Here is the link to my <a href="http://localhost:8888/notebooks/SUP2.1/Week1-Beginning%20Machine%20Learning%20with%20scikit-learn/Advertisement%20Notebook.ipynb" rel="nofollow noreferrer">notebook</a></p>