Add ScopedAllocator and ScopedAllocatorMgr.

A ScopedAllocator is a new memory allocator which manages slices of a large
tensor.  ScopedAllocatorMgr manages all scoped allocators per device.  The
ScopedAllocatorMgr comprises of multiple ScopedAllocatorContainers, one per
step id.  If a step terminates abnormally, the corresponding
ScopedAllocatorContainer will clean up all allocated memory in that step.

PiperOrigin-RevId: 189051914