docs: def get_nan_locs(self, **kwargs):
          """Gets the locations of nans in feature data and returns
          the coordinates in the matrix
          """
          if np.issubdtype(self.X.dtype, np.string_) or np.issubdtype(self.X.dtype, np.unicode_):
              mask = np.where( self.X == '' )
              nan_matrix = np.zeros(self.X.shape)
              nan_matrix[mask] = np.nan
  
          else:
              nan_matrix = self.X.astype(float)
  
          if self.y is None:
              return np.argwhere(np.isnan(nan_matrix))
          else:
              nan_locs = []
              for target_value in np.unique(self.y):
                  indices = np.argwhere(self.y == target_value)
                  target_matrix = nan_matrix[indices.flatten()]
                  nan_target_locs = np.argwhere(np.isnan(target_matrix))
                  nan_locs.append((target_value, nan_target_locs))
  
              return nan_locs