Fix accidentally disabled async mode for some t3k tests (#18381)

### Ticket
#18360

### Problem description
Recently we disabled async mode for single device, by ignoring
enable_async call for it, assuming multi-device customers make a call to
MeshDevice enable_async. However in some places including our test we
actually iterate over each individual device in the mesh and call
enable_async on it, which is being ignored

### What's changed
Make a single call to MeshDevice::enable_async instead of iterating over
individual devices and calling Device::enable_async for each one of them

### Checklist
- [ ] [All post commit CI
passes](https://github.com/tenstorrent/tt-metal/actions/runs/13553947437)
- [x] [T3K demo tests CI
passes](https://github.com/tenstorrent/tt-metal/actions/runs/13553950838)
- [x] New/Existing tests provide coverage for changes

conftest.py

@@ -342,13 +342,13 @@ def get_devices(request):
     elif "pcie_devices" in request.fixturenames:
         devices = request.getfixturevalue("pcie_devices")
     elif "mesh_device" in request.fixturenames:
-        devices = request.getfixturevalue("mesh_device").get_devices()
+        devices = [request.getfixturevalue("mesh_device")]
     elif "n300_mesh_device" in request.fixturenames:
-        devices = request.getfixturevalue("n300_mesh_device").get_devices()
+        devices = [request.getfixturevalue("n300_mesh_device")]
     elif "t3k_mesh_device" in request.fixturenames:
-        devices = request.getfixturevalue("t3k_mesh_device").get_devices()
+        devices = [request.getfixturevalue("t3k_mesh_device")]
     elif "pcie_mesh_device" in request.fixturenames:
-        devices = request.getfixturevalue("pcie_mesh_device").get_devices()
+        devices = [request.getfixturevalue("pcie_mesh_device")]
     else:
         devices = []
     return devices

models/demos/t3000/falcon40b/tests/test_falcon_end_to_end.py

@@ -526,8 +526,7 @@ def test_FalconCausalLM_end_to_end_with_program_cache(
     model_config = get_model_config(model_config_str, llm_mode, input_shape, num_devices)
     devices = t3k_mesh_device.get_devices()
     # Set async mode
-    for device in devices:
-        device.enable_async(async_mode)
+    t3k_mesh_device.enable_async(async_mode)
     compute_grid_size = devices[0].compute_with_storage_grid_size()
     if compute_grid_size.x < model_config["MAX_GRID_SIZE"][0] or compute_grid_size.y < model_config["MAX_GRID_SIZE"][1]:
         pytest.skip(f"Requires grid size of at least {model_config['MAX_GRID_SIZE']} to run")

tests/ttnn/distributed/test_multidevice_TG.py

@@ -1448,9 +1448,7 @@ def test_device_line_all_gather_8x4_data(mesh_device, cluster_axis: int, dim: in
     - Every device will have the shape: [4, 1, 32, 32]
     """
     if async_mode:
-        for i in mesh_device.get_device_ids():
-            device = mesh_device.get_device(i)
-            device.enable_async(True)
+        mesh_device.enable_async(True)
 
     (rows, cols), tile_size = mesh_device.shape, 32
     full_tensor = torch.zeros((1, 1, tile_size * rows, tile_size * cols), dtype=torch.bfloat16)