Setting ForwardDiff chunk size can error for sparse AD

[gdalle]

Problem:

• Since Force constant chunk size when specified in ForwardDiff #539, using a constant chunk size in AutoForwardDiff will error on input dimensions smaller than chunksize
• Since Revamp batch size handling #575, the batch size for sparse AD is picked after coloring and not before, so setting chunksize based on length(x) won't work (the true input dimension is the number of distinct colors, which is much smaller)