vector from that layer so that it can be reused in any computations.
Most layers won't use it but for some backward computations having it
allows them to be implemented more efficiently.

to just regular lists of parameters.

cuDNN.

need to deal with it.

confused when that name appeared on the command line.

we don't know what compiler we are dealing with.  If the test compile works
then we assume we have C++11.