Possibly undefined bridge in HLS and CPP backends

[thesps]

In the decision_function methods of HLS and C++ backends, the attribute self.bridge of the model is accessed to make the predictions. If compile hasn't already been called then this attribute won't exist and the method will fail in a hard-to-interpret way. Proposed fix: in the decision_function methods, check if self.bridge exists, and if not, then call compile.