Adding a test for swap kernel (#218)

Following up on conversation about copy optimization.

---------

Co-authored-by: Renat Idrisov <parsifal-47@users.noreply.github.com>

python/examples/test_swap.py

@@ -0,0 +1,44 @@
+import torch
+
+import triton
+import triton.language as tl
+
+# The purpose of this kernel and test is to catch incorrectly optimized kernels
+# where copy elimination happens erroneously in the absence of explicit memory allocation.
+# Such optimization bugs can result in incorrect behavior when swapping two arrays,
+# particularly when both arrays unintentionally end up with the same data due to
+# missing intermediate storage or mismanaged memory access.
+
+@triton.jit
+def swap_kernel(
+    x_ptr,  # *Pointer* to first inout vector.
+    y_ptr,  # *Pointer* to second inout vector.
+    BLOCK_SIZE: tl.constexpr,  # Number of elements each program should process.
+    # NOTE: `constexpr` so it can be used as a shape value.
+):
+    pid = tl.program_id(axis=0)  # We use a 1D launch grid so axis is 0.
+    block_start = pid * BLOCK_SIZE
+    offsets = block_start + tl.arange(0, BLOCK_SIZE)
+    x = tl.load(x_ptr + offsets)
+    y = tl.load(y_ptr + offsets)
+    tl.store(x_ptr + offsets, y)
+    tl.store(y_ptr + offsets, x)
+
+
+def swap(x: torch.Tensor, y: torch.Tensor):
+    n_elements = x.numel()
+    grid = lambda meta: (triton.cdiv(n_elements, meta["BLOCK_SIZE"]),)
+    swap_kernel[grid](x, y, BLOCK_SIZE=1024)
+
+
+def test(device):
+    torch.manual_seed(0)
+    size = 10240
+    x = torch.rand(size, device=device)
+    y = torch.rand(size, device=device)
+    assert not torch.equal(x, y)
+    x_ = x.clone()
+    y_ = y.clone()
+    swap(x, y)
+    assert torch.equal(x, y_)
+    assert torch.equal(y, x_)