docstr: Compute xpx and xpy using a single dot product
docstr_tok: ["Compute","xpx","and","xpy","using","a","single","dot","product"]
code: def _reset(self, idx):
      """Compute xpx and xpy using a single dot product"""
      _, wy, wx, _, not_missing = self._get_data(idx)
      nobs = not_missing.sum()
      xpx = wx.T @ wx
      xpy = wx.T @ wy
      return xpx, xpy, nobs
  
imports: from statsmodels.compat.numpy import lstsq
  from statsmodels.compat.pandas import Appender, Substitution, cache_readonly, call_cached_func, get_cached_doc
  from collections import namedtuple
  import numpy
  from pandas import DataFrame, MultiIndex, Series
  from scipy import stats
  from statsmodels.base import model
  from statsmodels.base.model import LikelihoodModelResults, Model
  from statsmodels.regression.linear_model import RegressionModel, RegressionResults
  from statsmodels.tools.validation import array_like, int_like, string_like
label_desc: Use image node_id 8 for calling the RollingWLS obj's underlying member method code with example usage: obj._reset(idx) and returns: xpx,  xpy,  nobs