Well even without deleting the Variables, so long as your loop is written in a way that doesn’t use results from the previous iteration, the following will happen. On each successive iteration, calculations will be made using new inputs, new targets and the parameters contained in build_feature_vector_using_a_CNN and lstm. As none of these have any computation graphs attached to them, the results of those calculations will build new graphs that have no connection to the graphs of the previous iteration.
Deleting all the Variables allows Python to garbage collect all the computation graphs, but assigning new results to the same python variable names does the same thing.