Hierarchical broadcast collective op.

Before this CL, we had a binary tree based implementation.  We would
essentially form an arbitrary, logical binary tree over all devices and then
send the tensor from each node to its children in the tree.

This change introduces a topology-aware hierarchical implementation.  First, we
perform a binary tree broadcast with only 1 device per machine.  Then for each
host, the device that participated in the first inter-machine broadcast is the
source for an intra-machine binary tree broadcast.

PiperOrigin-RevId: 206771821