Custom VJP and checkpointing (#541)

* Implement custom_vjp and checkpointing
* Add a dependency management primitive
* Change the eval order to deep branches first
* Add graph depth tracking to the array

tests/CMakeLists.txt

@@ -21,6 +21,7 @@ target_sources(tests PRIVATE
   autograd_tests.cpp
   blas_tests.cpp
   compile_tests.cpp
+  custom_vjp_tests.cpp
   creations_tests.cpp
   device_tests.cpp
   eval_tests.cpp