Added method .fit in the legacy preprocessor

bob/bio/base/transformers/preprocessor.py

@@ -37,3 +37,6 @@ class PreprocessorTransformer(TransformerMixin, BaseEstimator):
 
     def _more_tags(self):
         return {"stateless": True, "requires_fit": False}
+
+    def fit(self, X, y=None):
+        return self