code: def gradtrue(self, params):
      y, x = self.y, self.x
      return -x * 2 * (y - np.dot(x, params))[:, None]
  
imports: import numpy
  from numpy.testing import assert_allclose, assert_almost_equal
  import statsmodels.api
  from statsmodels.tools import numdiff
  from statsmodels.tools.numdiff import approx_fprime, approx_fprime_cs, approx_hess_cs, _approx_fprime_scalar, _approx_fprime_cs_scalar
label_desc: Use image node_id 3 for calling the TestDerivativeFun1 obj's underlying member method code with example usage: obj.gradtrue(params) and returns: unknown