[FEA] Add an _estimator_type attribute to estimators

[beckernick]

In scikit-learn, estimators have an _estimator_type attribute that indicates whether they are a classifier, regressor, or clusterer, etc.. This attribute is necessary for some scikit-learn API contracts, particularly around cross validation and meta-modeling. See here for the documentation.

For better compatibility with scikit-learn and higher-level tools enforcing the same contract, it would be useful if cuML estimators had this attribute as well. scikit-learn implements these in their Mixin classes. Because not all cuML estimators inherit from Mixins currently, perhaps these could be added in the estimator classes themselves as a start, and then handled via Mixin classes if/when additional estimators inherit from them.

Maybe something like:

class LogisticRegression(Base):
    def __init__(self, penalty='l2', tol=1e-4, C=1.0, fit_intercept=True,
                 class_weight=None, max_iter=1000, linesearch_max_iter=50,
                 verbose=False, l1_ratio=None, solver='qn',
                 handle=None):

        super(LogisticRegression, self).__init__(handle=handle,
                                                 verbose=verbose)
        ...
        self.C = C
        ...
       self._estimator_type = "classifier"
        ...

[beckernick]

Note that the Mixin classes already implement this attribute, so another approach would be to update estimators to inherit from the existing Mixins.

Perhaps these Mixins could be moved out of the metrics.base namespace and into the general common.base namespace? This would be consistent with sklearn

cuml/python/cuml/metrics/base.py

Lines 59 to 61 in f1f1c7f

	"""Mixin class for classifier estimators in"""
	_estimator_type = "classifier"

cuml/python/cuml/metrics/base.py

Lines 21 to 24 in f1f1c7f

	class RegressorMixin:
	"""Mixin class for regression estimators in"""
	_estimator_type = "regressor"

[beckernick]

After discussing with @dantegd , this seems like a good opportunity to update the existing Mixin classes