Unittests to demonstrate three bugs in flex mode:

 - forwardability is a property of a subgraph, not of the global buffer map
 - input tensors should never be forwardable, even when they
   come from another subgraph. That's because they might be outputs of the
   model, or inputs into another subgraph.
 - input tensors shouldn't be removed at the end, because they might be
   used by another subgraph.

PiperOrigin-RevId: 228228094