message: Optimize FIFO for depth=1
  Summary:
  Don't create the Switch and counter when the FIFO is made of a single buffer
  T11153
  Test Plan: All existing FIFO tests (Including the ones with depth=1) still pass
  Reviewers: #tensorflow, georgep
diff: @@ -89,6 +89,11 @@ class FifoOp : public PoplibsOpDef {
  }
  }
  + // If the FIFO can only store a single buffer then skip the counter creation
  + if (fifo_inst->depth() == 1) {
  + return sw_cases[0];
  + }
  +
  // Keep track of where in the buffer we are.
  auto counter = graph.addVariable(poplar::UNSIGNED_INT, {},
  GetDebugName(inst) + "/counter");
  