Single-controller Realm backend

[elliottslaughter]

This PR implements a single-controller Realm execution backend.

Feature overview:

• Single-controller (i.e., controller runs on one node)
• One task per dynamic graph node
  • Tasks are remote-launched if necessary (i.e., distributed execution should work, except for the caveats below)
• Tensors are allocated as instances in the memory nearest the assigned processor
• Most of the infrastructure for distributed execution is present, except:
  • Copies are not inserted, so data will not be moved between devices
  • PCG operators representing data movement (split, replicate, etc.) are not implemented

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This change is