fix #2108 (#2110)

* fix #2108

* clearner solution for fixing #2108

* bug fixed

Author: Junpeng Lao

pymc3/sampling.py

@@ -573,7 +573,8 @@ def init_nuts(init='ADVI', njobs=1, n_init=500000, model=None,
             progressbar=progressbar
         )  # type: pm.MeanField
         start = approx.sample(draws=njobs)
-        cov = approx.cov.eval()
+        stds = approx.gbij.rmap(np.diag(approx.cov.eval()))
+        cov = model.dict_to_array(stds)
         if njobs == 1:
             start = start[0]
     elif init == 'advi_map':