Function AddWhileGradientLoop

Creates the main backprop loop that computes the gradient of the loop associated with `while_ctx`. `grad_inputs` are the partial derivatives w.r.t. the loop outputs, i.e. the exit nodes. `backprop_execution_pred` is the predicate to use for the backprop loop (see AddBackPropLoopCounter()). The partial derivatives w.r.t. the loop inputs, i.e. the input loop vars, are returned in `grad_outputs`.