aten.empty is almost always fusible into its consumer, so we never CSE
it. This fixes a bug that looks like the following:

```py
torch.library.custom_op("_reinplacing::sin_cos", mutates_args={"out_sin", "out_cos"})
def sin_cos(x: torch.Tensor, out_sin: torch.Tensor, out_cos: torch.Tensor) -> None:
    out_sin.copy_(x.sin())
    out_cos.copy_(x.cos())

torch.compile
def f(x):
    out0 = torch.empty_like(x)
    out1 = torch.empty_like(x)
    sin_cos(x, out0, out1)
    return x.clone(), out0, out1

x = torch.randn(3, requires_grad=True)
f(x)
```

- cse would de-duplicate the empty nodes
- reinplacing would add an additional clone (because it can't write to
  both tensors at the same time)
- the "clone" is unnecessary - all reinplacing needed was an additional
  buffer, it doesn't matter what the values are.

We could attempt to fix this on the reinplacing side but this seemed
better as a partitioner heuristic and the reinplacing fix is a bit more
tricky (we'd need to identify that the op never reads from the empty
node).

Test Plan:
- new test (the old number was 27, the new number is 21, so this PR
  helped).

ghstack-source-id: 37a5d74
Pull Request resolved: #134703

aten.empty is almost always fusible into its consumer, so we never CSE
it. This fixes a bug that looks like the following:

```py
@torch.library.custom_op("_reinplacing::sin_cos", mutates_args={"out_sin", "out_cos"})
def sin_cos(x: torch.Tensor, out_sin: torch.Tensor, out_cos: torch.Tensor) -> None:
    out_sin.copy_(x.sin())
    out_cos.copy_(x.cos())

@torch.compile
def f(x):
    out0 = torch.empty_like(x)
    out1 = torch.empty_like(x)
    sin_cos(x, out0, out1)
    return x.clone(), out0, out1

x = torch.randn(3, requires_grad=True)
f(x)
```

- cse would de-duplicate the empty nodes
- reinplacing would add an additional clone (because it can't write to
  both tensors at the same time)
- the clone lowers into a new buffer + a copy_ kernel
- the copy_ kernel is unnecessary because "empty" is special - all reinplacing needed was an additional
  buffer, it doesn't matter what the values are.

We could attempt to fix this on the reinplacing side but this seemed
better as a partitioner heuristic and the reinplacing fix is a bit more
tricky (we'd need to identify that the op never reads from the empty
node).

Test Plan:
- new test (the old number was 27, the new number is 21, so this PR
  helped).

Pull Request resolved: pytorch#134703
Approved by: https://github.com/yf225
ghstack dependencies: pytorch#134466, pytorch#134490, pytorch#134491