forwardDType shouldn't set optional tensor

Related to: aidge_backend_cpu#58 (closed)

Problem seems to be due to the change of setDataType to forwardDType in the compile method (cf. 772300d6) The failing test use FC with no bias, forwardDType(float32) set every input to float32 creating a tensor if it does not exist yet.. Problem is that it then create an empty tensor for the bias which lead to an error in the implementation of FC.