Fix test_new_all_gather.py regressions caused by API unification betw…

…een Device/MeshDevice (#16836)

### Ticket
Link to Github Issue

### Problem description
After Device / MeshDevice API unification merge, one of the programs is
getting a core specified that is not one of its active eth cores. I
triaged and reverted the bad changes that cause this regression.

### What's changed
The change that I made seemed to have altered the device ordering
contained within MeshDevice. These new CCL tests seem to be sensitive to
the ordering of devices.

### Checklist
- [x] Post commit CI passes:
https://github.com/tenstorrent/tt-metal/actions/runs/12819606844
- [x] T3000 Tests:
https://github.com/tenstorrent/tt-metal/actions/runs/12819611934

tt_metal/distributed/mesh_device.cpp

@@ -161,7 +161,8 @@ MeshDevice::~MeshDevice() {}
 
 IDevice* MeshDevice::get_device_index(size_t device_index) const {
     TT_FATAL(device_index >= 0 and device_index < num_devices(), "Invalid device index");
-    return this->get_devices().at(device_index);
+    const auto& devices = scoped_devices_->get_devices();
+    return devices.at(device_index);
 }
 
 IDevice* MeshDevice::get_device(chip_id_t physical_device_id) const {
@@ -671,7 +672,7 @@ MemoryBlockTable MeshDevice::get_memory_block_table(const BufferType& buffer_typ
 MeshSubDeviceManagerId MeshDevice::mesh_create_sub_device_manager(
     tt::stl::Span<const SubDevice> sub_devices, DeviceAddr local_l1_size) {
     MeshSubDeviceManagerId mesh_sub_device_manager_id(*this);
-    const auto& devices = this->get_devices();
+    const auto& devices = scoped_devices_->get_devices();
     for (uint32_t i = 0; i < devices.size(); i++) {
         auto* device = devices[i];
         auto& sub_device_manager_id = mesh_sub_device_manager_id.sub_device_manager_ids[i];
@@ -688,7 +689,7 @@ MeshSubDeviceManagerId MeshDevice::mesh_create_sub_device_manager(
 std::tuple<MeshSubDeviceManagerId, SubDeviceId> MeshDevice::mesh_create_sub_device_manager_with_fabric(tt::stl::Span<const SubDevice> sub_devices, DeviceAddr local_l1_size) {
     MeshSubDeviceManagerId mesh_sub_device_manager_id(*this);
     SubDeviceId fabric_sub_device_id;
-    const auto& devices = this->get_devices();
+    const auto& devices = scoped_devices_->get_devices();
     for (uint32_t i = 0; i < devices.size(); i++) {
         auto* device = devices[i];
         auto& sub_device_manager_id = mesh_sub_device_manager_id.sub_device_manager_ids[i];
@@ -704,7 +705,7 @@ std::tuple<MeshSubDeviceManagerId, SubDeviceId> MeshDevice::mesh_create_sub_devi
 }
 
 void MeshDevice::mesh_load_sub_device_manager(MeshSubDeviceManagerId mesh_sub_device_manager_id) {
-    const auto& devices = this->get_devices();
+    const auto& devices = scoped_devices_->get_devices();
     for (uint32_t i = 0; i < devices.size(); i++) {
         auto* device = devices[i];
         auto sub_device_manager_id = mesh_sub_device_manager_id.sub_device_manager_ids[i];
@@ -713,13 +714,12 @@ void MeshDevice::mesh_load_sub_device_manager(MeshSubDeviceManagerId mesh_sub_de
     }
 }
 void MeshDevice::mesh_clear_loaded_sub_device_manager() {
-    const auto& devices = this->get_devices();
-    for (auto* device : devices) {
+    for (auto* device : scoped_devices_->get_devices()) {
         device->push_work([device]() { device->clear_loaded_sub_device_manager(); });
     }
 }
 void MeshDevice::mesh_remove_sub_device_manager(MeshSubDeviceManagerId mesh_sub_device_manager_id) {
-    const auto& devices = this->get_devices();
+    const auto& devices = scoped_devices_->get_devices();
     for (uint32_t i = 0; i < devices.size(); i++) {
         auto* device = devices[i];
         auto sub_device_manager_id = mesh_sub_device_manager_id.sub_device_manager_ids[i];
@@ -729,13 +729,13 @@ void MeshDevice::mesh_remove_sub_device_manager(MeshSubDeviceManagerId mesh_sub_
 }
 
 void MeshDevice::mesh_set_sub_device_stall_group(tt::stl::Span<const SubDeviceId> sub_device_ids) {
-    for (auto* device : this->get_devices()) {
+    for (auto* device : scoped_devices_->get_devices()) {
         device->push_work([device, sub_device_ids=std::vector<SubDeviceId>(sub_device_ids.begin(), sub_device_ids.end())]() { device->set_sub_device_stall_group(sub_device_ids); });
     }
 }
 
 void MeshDevice::mesh_reset_sub_device_stall_group() {
-    for (auto* device : this->get_devices()) {
+    for (auto* device : scoped_devices_->get_devices()) {
         device->push_work([device]() { device->reset_sub_device_stall_group(); });
     }
 }