Returns the tensors to pass as captured inputs to `body_grad_graph`. `body_grad_graph` may have external references to: 1. Its outer graph containing the input gradients. These are left as-is. 2. Accumulators captured from the forward-pass graph. These should have been added as `while_op
(body_graph, body_grad_graph, while_op)
source not stored for this graph (policy: none)