objax.Vectorize batch_axis argument for multiple arguments default must be adaptively sized.

[JeremyNixon]

At present the default is set for a single argument. If there are two arguments, ex:
objax.Vectorize(augment, objax.VarCollection())
batch_axis=(0) as a default leads to an error.
The default batch_axis must be set to , batch_axis=(0, 0) for two arguments, and in general have one 0 for each argument.