Optional inputs handling & NoBias attribute inconsistency

Operator FC has a NoBias attribute, but Conv and other operators do not. We should decide on a consistent mechanism to handle bias.

Futhermore, right now, outputDimsForwarded() won't work with FC without bias, because the function checks that all the inputs must be non-empty. But when the NoBias attribute is true, the bias Producer is set to have a size of 0 (a.k.a. empty)!