Add support for the TensorFlow 2.0 TensorShape behavior (more user-friendly...

Add support for the TensorFlow 2.0 TensorShape behavior (more user-friendly and less bug-prone). Also adds compatibility utilities to help convert old code to the new behavior:

`enable_v2_tensorshape` & `disable_v2_tensorshape`: toggle between new/old behavior.
`dimension_value` & `dimension_at_index`: utilities to write code that is compatible with both behaviors seamlessly.

PiperOrigin-RevId: 218048361