[NNVM][TEST] Test against numerical grad (#1505)

* [NNVM][TEST] Numerical gradient testing

* [NNVM][TEST] Make some tests a little faster

* Fix the failing test_top_level3

* Target exclusion for the check_function

* Try to ignore singularities

* grad_input_vars now can't contain shapes

* Don't pass unnecessary grad_input_vars to check_function

* Multiple outputs; fixes; testing of check_function

* Use numerical_grads_params to pass parameters to numgrad checker

* Fail when no action is requested excplicitly

* Pass additional params to functions

* Silence the linter issue

* Simplified numgrad checking

* Improved docs for check_function

* Fixed the error message when no dtype is provided

* Several fixes

* Tests with shape/dtype inference for inputs

* Don't check dense's grads on cuda

* Raise an error if output dtypes haven't been inferred

* Moved shape/dtype inference into a separate function; use float32 as fallback

* Remove redundant dtype=float32

* Fix multiple outputs

* Use check_function in the rest of the test_top_level1