Support for SLEP010

This requires setting a public `n_features_in_` attribute as part
of the fit() logic.

For details, see:
https://scikit-learn-enhancement-proposals.readthedocs.io/en/latest/slep010/proposal.html

Author: perimosocordiae

.gitignore

@@ -8,4 +8,5 @@ htmlcov/
 .pytest_cache/
 doc/auto_examples/*
 doc/generated/*
-venv/
+venv/
+.vscode/

metric_learn/base_metric.py

@@ -166,12 +166,16 @@ def _prepare_inputs(self, X, y=None, type_of_inputs='classic',
     self._check_preprocessor()
 
     check_is_fitted(self, ['preprocessor_'])
-    return check_input(X, y,
+    outs = check_input(X, y,
                        type_of_inputs=type_of_inputs,
                        preprocessor=self.preprocessor_,
                        estimator=self,
                        tuple_size=getattr(self, '_tuple_size', None),
                        **kwargs)
+    # Conform to SLEP010
+    if not hasattr(self, 'n_features_in_'):
+      self.n_features_in_ = (outs if y is None else outs[0]).shape[1]
+    return outs
 
   @abstractmethod
   def get_metric(self):