Create kNN docs draft

Author: xhlulu

doc/python/ml-knn.md

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+## K-Nearest Neighbors (kNN)
+
+How to visualize the K-Nearest Neighbors (kNN) algorithm using scikit-learn.
+
+
+### Binary Probability Estimates with `go.Contour`
+
+```python
+import numpy as np
+from sklearn.datasets import make_moons
+from sklearn.neighbors import KNeighborsClassifier
+import plotly.express as px
+import plotly.graph_objects as go
+
+X, y = make_moons(noise=0.3, random_state=0)
+
+# Create a mesh grid on which we will run our model
+x_min, x_max = X[:, 0].min() - margin, X[:, 0].max() + margin
+y_min, y_max = X[:, 1].min() - margin, X[:, 1].max() + margin
+xrange = np.arange(x_min, x_max, mesh_size)
+yrange = np.arange(y_min, y_max, mesh_size)
+xx, yy = np.meshgrid(xrange, yrange)
+
+# Create classifier, run predictions on grid
+clf = neighbors.KNeighborsClassifier(15, weights='uniform')
+clf.fit(X, y)
+Z = clf.predict_proba(np.c_[xx.ravel(), yy.ravel()])[:, 1]
+Z = Z.reshape(xx.shape)
+
+fig = px.scatter(X, x=0, y=1, color=y.astype(str))
+fig.add_trace(
+    go.Contour(
+        x=xrange, 
+        y=yrange, 
+        z=Z, 
+        showscale=False,
+        colorscale=['Blue', 'Red'],
+        opacity=0.4
+    )
+)
+```
+
+### Multi-class classification with `px.data` and `go.Heatmap`
+
+```python
+import numpy as np
+from sklearn.neighbors import KNeighborsClassifier
+import plotly.express as px
+import plotly.graph_objects as go
+
+mesh_size = .02
+margin = 1
+
+df = px.data.iris()
+X = df[['sepal_length', 'sepal_width']]
+y = df.species_id
+
+# Create a mesh grid on which we will run our model
+l_min, l_max = df.sepal_length.min() - margin, df.sepal_length.max() + margin
+w_min, w_max = df.sepal_width.min() - margin, df.sepal_width.max() + margin
+lrange = np.arange(l_min, l_max, mesh_size)
+wrange = np.arange(w_min, w_max, mesh_size)
+ll, ww = np.meshgrid(lrange, wrange)
+
+# Create classifier, run predictions on grid
+clf = KNeighborsClassifier(15, weights='distance')
+clf.fit(X, y)
+Z = clf.predict(np.c_[ll.ravel(), ww.ravel()])
+Z = Z.reshape(ll.shape)
+
+fig = px.scatter(df, x='sepal_length', y='sepal_width', color='species')
+fig.update_traces(marker_size=10, marker_line_width=1)
+fig.add_trace(
+    go.Heatmap(
+        x=lrange, 
+        y=wrange, 
+        z=Z, 
+        showscale=False,
+        colorscale=[[0.0, 'blue'], [0.5, 'red'], [1.0, 'green']],
+        opacity=0.25
+    )
+)
+```
+
+### Visualizing kNN Regression
+
+```python
+from sklearn.neighbors import KNeighborsRegressor
+import plotly.express as px
+import plotly.graph_objects as go
+
+df = px.data.tips()
+X = df.total_bill.values.reshape(-1, 1)
+
+knn_dist = KNeighborsRegressor(10, weights='distance')
+knn_uni = KNeighborsRegressor(10, weights='uniform')
+knn_dist.fit(X, df.tip)
+knn_uni.fit(X, df.tip)
+
+x_range = np.linspace(X.min(), X.max(), 100)
+y_dist = knn_dist.predict(x_range.reshape(-1, 1))
+y_uni = knn_uni.predict(x_range.reshape(-1, 1))
+
+fig = px.scatter(df, x='total_bill', y='tip', color='sex', opacity=0.65)
+fig.add_traces(go.Scatter(x=x_range, y=y_uni, name='Weights: Uniform'))
+fig.add_traces(go.Scatter(x=x_range, y=y_dist, name='Weights: Distance'))
+```
+
+### Reference
+
+Learn more about `px`, `go.Contour`, and `go.Heatmap` here:
+* https://plot.ly/python/plotly-express/
+* https://plot.ly/python/heatmaps/
+* https://plot.ly/python/contour-plots/
+
+This tutorial was inspired by amazing examples from the official scikit-learn docs:
+* https://scikit-learn.org/stable/auto_examples/neighbors/plot_regression.html
+* https://scikit-learn.org/stable/auto_examples/neighbors/plot_classification.html
+* https://scikit-learn.org/stable/auto_examples/classification/plot_classifier_comparison.html