* add support for subgraphs.

* fix.

* fix.

* Fix compilation error

* Fix compilation error

* add comments.

* update comments.

* Sanity check on subgraphs when creating IndexedGraph

* avoid the overhead of sanity check.

* Stop using non-recursive DFS

* Trigger CI

* trigger CI

* [NNVM]Activations support added in Keras Frontend

* Helper for ELU added

* All activations test cases clubbed to one

[NNVM] Fix softmax gradient

************* Module nnvm.top.nn
E:141, 8: Assigning to function call which doesn't return (assignment-from-no-return)

* update tvm submodule

* Add count_include_pad support to avg_pool

Not able link build/src/top/tensor/*.d.
       Hence don't compile nnvm for a change in tvm/topi headers.

Better to check Infer result with topi results at build time instead of leaving to a runtime error. (#476)