NoOp scheduler claims zero-dimensional non-trivial fusions

[jacobhinkle]

I'm not sure if this is intended behavior, but it led to some small confusion leading to reordering the ExpressionEvaluator scheduler before NoOp in #2209. I'm posting this issue since it is confusing for a NoOp scheduler to compute an op.

The following test "fails":

TEST_F(NVFuserTest, ZeroDimAdd) {
  auto fusion = std::make_unique<Fusion>();
  FusionGuard fg(fusion.get());

  auto tv0 = makeSymbolicTensor(0, DataType::Float);
  auto tv1 = makeSymbolicTensor(0, DataType::Float);
  auto tv2 = add(tv0, tv1);

  fusion->addInput(tv0);
  fusion->addInput(tv1);
  fusion->addOutput(tv2);

  at::Tensor t0 = at::randn({}, at::kFloat).cuda();
  at::Tensor t1 = at::randn({}, at::kFloat).cuda();
  at::Tensor out_ref = t0 + t1;

  FusionExecutorCache fec(std::move(fusion));
  auto out = fec.runFusionWithInputs({t0, t1});

  const FusionKernelRuntime* runtime = fec.getMostRecentKernelRuntime();
  EXPECT_FALSE(runtime->isSegmented());
  const std::vector<FusionExecutor>& executors = runtime->executors();
  EXPECT_EQ(executors.size(), 1);
  // Verify that fusion compilation was not skipped.
  EXPECT_TRUE(executors.front().hasCompiledKernel());

  EXPECT_EQ(
      runtime->schedulerHeuristics()->heuristicsList().front()->heuristic(),
      ScheduleHeuristic::PointWise);
  /*
Expected equality of these values:
  runtime->schedulerHeuristics()->heuristicsList().front()->heuristic()
    Which is: no_op
  ScheduleHeuristic::PointWise
    Which is: pointwise
  */

  EXPECT_TRUE(at::allclose(out[0], out_ref));
}

Even though the no-op scheduler claims the segment, it does result in a working fusion and the result is correct. Here is the kernel:

__global__ void nvfuser_no_op_f0_c1_r0_g0(Tensor<float, 0, 0> T0, Tensor<float, 0, 0> T1, Tensor<float, 0, 0> T2) {
  T2[0]
    = T0[0]
    + T1[0];
}

Segmenter logging doesn't seem to give any clues as to why NoOp accepts the fusion:

***Runtime***: Try to schedule fusion un-segmented:

Scheduler _expr_eval_ ***rejected*** because : Fusion must contain a single expression of type MatmulOp
***Accepted*** as: no_op

[jacobhinkle]

This is actually the intended behavior of the NoOp scheduler. We just need to be more careful to exclude ops that produce reductions to zero-dimensional outputs, like matmuls with all 1D inputs. I'll do that in #2236 and close this.