The first command buffer takes the first 10% of the graph nodes and the
rest is split evenly between the extra command buffers. A fusion cannot
span two command buffers, so a boundary that falls inside a fusable
group runs that group unfused, and the output then depends on the node
count of the graph.

Move each boundary to the end of the fusable group it falls into, so
the encoders fuse the same nodes as with a single command buffer.