Initial checkin for outside_compilation. Adds a new attribute for...

Initial checkin for outside_compilation. Adds a new attribute for encapsulating XLA subgraphs that will in the future be used to mark some Ops in the subgraph as 'outside_compilation' meaning they will be run as interpreted TensorFlow via a callout from a compiled XLA subgraph.

This is the first of a sequence of checkins. It adds new types of edges entering and leaving the subgraphs, suitable for send/recv between a compiled XLA subgraph and the 'host', i.e., uncompiled TensorFlow.

For now no code sets the new 'outside_compilation' attributes, and the Ops to perform the send/recv are not present in the codebase; these will follow in subsequent checkins.

PiperOrigin-RevId: 179591853