[Autograd] Variables cannot be dereferenced

[mratsim]

Due to the following template, variables cannot be dereferenced.

https://github.com/mratsim/Arraymancer/blob/fc4ad528f6afcd377c16ff99f19151a0f5e46f89/src/autograd/ag_accessors.nim#L18-L28

This forces to use casting instead of addr v[]

https://github.com/mratsim/Arraymancer/blob/fc4ad528f6afcd377c16ff99f19151a0f5e46f89/src/autograd/ag_data_structure.nim#L116-L120

Solution is probably easy. Dispatch to system.[] when varargs = 0.