(1) Registry now uses std::function for more flexible use cases.
(2) dropout adds an "is_test" keyword.
(3) Making all gradient registered via C++. Python still provides gradient wrapper.
TODO item is to make the autograd SSA in C++ if possible. Problem is if we want to dynamically
register python gradients we will be sort of screwed because in c++ things are registered
via static variables.
(1) various bugfixes.
(2) Tensor is now a class independent from its data type. This allows us
to write easier type-independent operators.
(3) code convention changes a bit: dtype -> T, Tensor<*Context> -> Tensor* alias.
(4) ParallelNet -> DAGNet to be more consistent with what it does.
(5) Caffe's own flags library instead of gflags.
(6) Caffe's own logging library instead of glog, but glog can be chosen with
compile-time definition -DCAFFE2_USE_GOOGLE_GLOG. As a result, glog macros
like CHECK, DCHECK now have prefix CAFFE_, and LOG(*) now becomes
CAFFE_LOG_*.
(7) an optional protobuf inclusion, which can be chosen with USE_SYSTEM_PROTOBUF
in build_env.py.
