Checkpointable: Utility to split a dependency for saving/loading

Checkpointable: Utility to split a dependency for saving/loading

Useful when a single op produces Tensors which should each be saved under
different objects (or when Tensors saved with many different objects need to be
restored together as inputs to a single op).

I plan to use this for cuDNN RNN saving, which currently relies heavily on name
matching to allow cuDNN cells to be swapped out for CPU-compatible RNN
cells. This change has just the utility and some tests for it.

PiperOrigin-RevId: 191509664