adding example for memory wrapper
A short cleanup
Merge pull request #3795 from dsullivan7/svm_doc

[MRG+1] adding example for memory wrapper for load_svm_light
recovering deleted variable
MAINT remove deprecated loss in gradient boosting
Handle_unknown option to OneHotEncoder

OneHotEncoder fails with a error that is not helpful if a missing
categorical feature is present during transformation.
Add handle_unknown with an "error" and an "ignore" option.
DOC add missing public function into the references
DOC: Remove extra # from url in fit.

The extra # messes with the rst parser which produces an invalid URL where the ".html" suffix is missing.
Merge pull request #3797 from Titan-C/clean

A short cleanup
Merge pull request #3801 from cmd-ntrf/typo_vbgmm

DOC: Remove extra # from url in fit.
DOC: Make the comment slightly clearer
Merge pull request #3800 from MechCoder/unknown_transform

Handle_unknown option to OneHotEncoder
COSMIT: Minor opt in pairwise_distances_argmin_min
ENH: Birch, first commit
add example for birch + make it more pythonic
Made the following changes
1. Improved documentation.
2. Added sanity test.
3. Moved example to a separate folder
4. Optimised compuation of centroids.
5. Added underscore to public attributes.
ENH: Made the following changes
1. Added partial_fit and predict methods
2. Inherit from ClusterMixin from fit_predict methods
3. Removed inplace modification of X and self.copy
4. Added final global clustering and n_clusters param.
OPT: Prevent check_array being called twice in check_pairwise
FIX: Fix repeated checking_pairwise in pairwise_distances_argmin_min