Fix wrong inputs/model placement when using a single core (#725)

fix

optimum/neuron/modeling_traced.py

@@ -53,6 +53,7 @@
     NEURON_COMPILER_VERSION = get_neuroncc_version()
 
 if is_neuronx_available():
+    import torch_neuronx
     from torch_neuronx import move_trace_to_device
 
     NEURON_COMPILER_TYPE = "neuronx-cc"
@@ -127,8 +128,12 @@ def load_model(
         if path.is_file():
             model = torch.jit.load(path)
             # For non-inlined models, send the module manually to device. This is important for weights/neff non-inlined module since when loading the module, the neff is automatically moved to Neuron but not the weights. We need to move the weights to Neuron as well manually to avoid great host to device IO penalty.
-            if is_neuronx_available() and to_neuron:
-                move_trace_to_device(model, device_id)
+            if is_neuronx_available():
+                torch_neuronx.experimental.set_neuron_cores(
+                    model, start_nc=0, nc_count=1
+                )  # The inputs are allocated to nc:0 by default, this line ensures both input tensors and the model are on the same core.
+                if to_neuron:
+                    move_trace_to_device(model, device_id)
             return model
 
     def replace_weights(self, weights: Optional[Union[Dict[str, torch.Tensor], torch.nn.Module]] = None):