Add HloModuleGroup abstraction.

This CL adds HloModuleGroup which is a simple container of HLO modules. The module group gathers together HLO modules which are built to run concurrently across multiple devices. This cl just adds the container class. Later CLs will tie this into other parts of XLA including adding HloModuleGroup HLO passes which operate on an entire module group.

PiperOrigin-RevId: 212821390