code: def fit(self, X, y=None):
      X = check_X(X)
      self.variables_ = self.variables
      self.feature_names_in_ = list(X.columns)
      self.n_features_in_ = X.shape[1]
      return self
  
imports: import numpy
  import pytest
  from sklearn.exceptions import NotFittedError
  from sklearn.impute import SimpleImputer
  from sklearn.pipeline import Pipeline
  from sklearn.preprocessing import PolynomialFeatures
  from feature_engine._base_transformers.mixins import GetFeatureNamesOutMixin
  from feature_engine.dataframe_checks import check_X
label_desc: Use image node_id 2 for calling the MockCreator obj's underlying member method code with example usage: obj.fit(X, y) and returns: self