class_docstr: Simple selector that subsets a DataFrame to the columns of interest
  This allows a Pipeline that applies different transformations to subsets of columns,
     and unions that result
code: def fit(self, X, y=None):
      return self
  
file_path: greykite/greykite/sklearn/transform/column_selector.py
imports: import pandas
  from sklearn.base import BaseEstimator
  from sklearn.base import TransformerMixin
label_desc: Use image node_id 2 for calling the ColumnSelector obj's underlying member method code with example usage: obj.fit(X, y) and returns: self