FIX: Make pd import local in DataFrameTransformer (#1017)

DataFrameTransformer requires pandas but we don't want to have a
dependency on pandas. Therefore, the pandas import should be local.

Back when I wrote this, I thought it would be sufficient to import
pandas at a class level, but that is incorrect. Now, pandas is important
inside the method bodies.

For most skorch users, this should make no difference. But it will
allow skorch users who do use something from helpers.py to avoid having
to install pandas.

skorch/helper.py

@@ -370,8 +370,6 @@ class DataFrameTransformer(BaseEstimator, TransformerMixin):
     contains 1 column.
 
     """
-    import pandas as pd
-
     def __init__(
             self,
             treat_int_as_categorical=False,
@@ -399,6 +397,8 @@ def _check_dtypes(self, df):
           If a wrong dtype is found.
 
         """
+        import pandas as pd
+
         if 'X' in df:
             raise ValueError(
                 "DataFrame contains a column named 'X', which clashes "
@@ -408,7 +408,7 @@ def _check_dtypes(self, df):
         wrong_dtypes = []
 
         for col, dtype in zip(df, df.dtypes):
-            if isinstance(dtype, self.pd.api.types.CategoricalDtype):
+            if isinstance(dtype, pd.api.types.CategoricalDtype):
                 continue
             if np.issubdtype(dtype, np.integer):
                 continue
@@ -447,6 +447,8 @@ def transform(self, df):
           respective column names as keys.
 
         """
+        import pandas as pd
+
         self._check_dtypes(df)
 
         X_dict = {}
@@ -455,7 +457,7 @@ def transform(self, df):
         for col, dtype in zip(df, df.dtypes):
             X_col = df[col]
 
-            if isinstance(dtype, self.pd.api.types.CategoricalDtype):
+            if isinstance(dtype, pd.api.types.CategoricalDtype):
                 x = X_col.cat.codes.values
                 if self.int_dtype is not None:
                     x = x.astype(self.int_dtype)