* fix dygraph double grad dtype error when calling for high differential senario

* reinvoke ci

* add test for partial_engine.cc

* init functional jacobian api

* finish test with dtype float32

* add float64 test case

* polish code

* use atol=1e-5 with dtype float64

* fix for ci

* set timeout for test_jacobian

* polish API docstring

* modify docstring