Fixes #498 Modifies ModelVisualizers to wrap Pipeline objects

[lwgray]

Make estimator attribute a property so that it can be examined if it …

This PR fixes #498 which requested that the model visualizer be modified to be able to wrap pipeline objects.

I have made the following changes:

1. Made estimator attribute a property that sets the estimator to the final_estimator if pipeline object is detected. This that the final_estimator is not a transformer.

Sample Code and Plot

from sklearn.datasets import load_iris
from sklearn.linear_model import LogisticRegression
from sklearn.model_selection import train_test_split as tts

from yellowbrick.classifier import ConfusionMatrix

iris = load_iris()
X = iris.data
y = iris.target
classes = iris.target_names

X_train, X_test, y_train, y_test = tts(X, y, test_size=0.2)

model = Pipeline([
    ('minmax', MinMaxScaler()), 
    ('tree', LogisticRegression(multi_class="auto", solver="liblinear")), 
]) 

iris_cm = ConfusionMatrix(
    model, classes=classes,
    label_encoder={0: 'setosa', 1: 'versicolor', 2: 'virginica'}
)

iris_cm.fit(X_train, y_train)
iris_cm.score(X_test, y_test)
iris_cm.show()

TODOs and questions

Still to do:

• [ ] Write Test for each visualizer with models within a pipeline

CHECKLIST

• [x] Is the commit message formatted correctly?

• [X] Included a sample plot to visually illustrate your changes?
• [ ] Do all of your functions and methods have docstrings?
• [ ] Have you added/updated unit tests where appropriate?
• [ ] Have you updated the baseline images if necessary?
• [ ] Have you run the unit tests using pytest?
• [ ] Is your code style correct (are you using PEP8, pyflakes)?
• [ ] Have you documented your new feature/functionality in the docs?

• [ ] Have you built the docs using make html?

[codecov[bot]]

Codecov Report

Merging #1245 (50f17be) into develop (e339ce2) will decrease coverage by 0.03%. The diff coverage is 76.92%.

@@             Coverage Diff             @@
##           develop    #1245      +/-   ##
===========================================
- Coverage    90.57%   90.54%   -0.04%     
===========================================
  Files           92       92              
  Lines         5209     5222      +13     
===========================================
+ Hits          4718     4728      +10     
- Misses         491      494       +3     

Impacted Files	Coverage Δ
yellowbrick/base.py	91.17% <76.92%> (-1.19%)	:arrow_down:

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