Missing input dimensions change check in operators

Once the dimensions are forwarded, they are not supposed to change between call to forward(), because once the output dims of an operator are set, they are not recomputed at each forward pass.

So, if after a forwardDims() call or a forward pass, the input dimensions change, this current result is undefined behavior (likely, a segfault).

While this is not the typical case, this issue must be addressed properly as this is a significant design flaw for now.