title: Attributes of base estimators in Regressor Chain
body: <p>I am solving multi-output regression problem using RegressorChain in Scikit Learn, but after fitting the model i need to retrieve the fitted model base estiamtor to access the estimator attributes .
  I tried to access the attributes of base estimators in RegerssorChain in Scikit Learn, but I couldn't.</p>
  <p>The following is the code I used for this.</p>
  <pre><code>linreg = LinearRegression() 
  
  wrapper=RegressorChain(linreg)
  
  fit_model=wrapper.fit(X_train, y_train)
  
  base_estimator_para=RegressorChain.get_params(fit_model, deep=True)
  
  base_estimator_linreg=base_estimator_para['base_estimator']
  
  print(base_estimator_linreg.coef_)
  </code></pre>
  <p>The last line returns an error that 'LinearRegression' object has no attribute 'coef_', but LinearRegression has the attribute 'coef_'.</p>
  <p>The function &quot;base_estimator_para&quot; returns a dicitonary contains objects including the base estimator itself.</p>
  <p><code>base_estimator_para['base_estimator']</code> returns the base estimator object, so using<br />
  <code>base_estimator_linreg.coef_</code> is supposed to return the coefficients of the fitted linear regression model.</p>