I think the check is there just to provide that helper warning message. If you’re not changing the last dimension of X you could pass that info with x = pm.MutableData(..., shape=(None, 12)), for example, where with 12 your restrict yourself to always have that size for the last dimension and None allows the other dim to vary.
Then the constant_fold would work?
CC @bwengals, not sure if there’s any reason to require static shape?