* Factor python dependency out of interpreter
* Remove NO_PYTHON for the autograd engine
If there is no python bindings, then a default Engine is constructed
the first time it is requested.
If the python libraries are loaded, then they override the default
accessor and the default engine becomes a python Engine.
Note: it is possible for two engines to be generated if a non-python
one gets created before the python bindings are loaded. This case
is rare, and just results in additional threads being spawned.
* Fixing AlexNet test which is skipped in CI
