[SYCL][Graph] Breadth-first schedule

[Wee-Free-Scot]

The intent is to change the "reversed depth-first" scheduling/ordering (as implemented in topology_sort) to a strict breadth-first scheduling/ordering. The schedule is now computed by building a multimap of nodes in layers.

Draft documentation wording for the change of schedule ordering:

"Breadth-first traversal when scheduling a graph maximizes the number of independent kernels that are submitted concurrently, which enables more efficient work scheduling on massively parallel devices by overlapping kernel launch overheads with independent computation and data movement actions."

A side-effect is to change the trigger that creates the schedule so the executable graph no longer needs an internal reference to the modifiable graph from which it was created. The schedule is now created during the constructor for the executable graph rather than a later call to a method on the modifiable graph.

[EwanC]

I have a bunch of superficial comments on this PR, and I'm seeing a build fail, but fundamentally I'm okay with using the breadth-first approach.

A quick description (can just append to PR description) of why this algorithm is preferable would be useful though, since we may want to document this implementation detail in #183 and justify why it's the default to other internal & external stakeholders.