[HLSCustomOp] add i/o shape retrieval fxns to HLSCustomOp base

src/finn/custom_op/fpgadataflow/__init__.py

@@ -299,3 +299,15 @@ compilation transformations?
     @abstractmethod
     def pragmas(self):
         pass
+
+    def get_folded_input_shape(self):
+        raise Exception("get_folded_input_shape not implemented for this op")
+
+    def get_folded_output_shape(self):
+        raise Exception("get_folded_output_shape not implemented for this op")
+
+    def get_instream_width(self):
+        raise Exception("get_instream_width not implemented for this op")
+
+    def get_outstream_width(self):
+        raise Exception("get_outstream_width not implemented for this op")