refactor(processor): rename internal device variable for clarity (#1857)

- Changed the internal device variable from `_device` to `tensor_device` for improved readability and consistency.
- Updated references throughout the class to reflect the new variable name.

src/lerobot/processor/device_processor.py

@@ -50,8 +50,8 @@ class DeviceProcessorStep(ProcessorStep):
     }
 
     def __post_init__(self):
-        self._device: torch.device = get_safe_torch_device(self.device)
-        self.device = self._device.type  # cuda might have changed to cuda:1
+        self.tensor_device: torch.device = get_safe_torch_device(self.device)
+        self.device = self.tensor_device.type  # cuda might have changed to cuda:1
         self.non_blocking = "cuda" in str(self.device)
 
         # Validate and convert float_dtype string to torch dtype
@@ -73,7 +73,7 @@ class DeviceProcessorStep(ProcessorStep):
         Otherwise, it moves to the configured device.
         """
         # Determine target device
-        if tensor.is_cuda and self._device.type == "cuda":
+        if tensor.is_cuda and self.tensor_device.type == "cuda":
             # Both tensor and target are on GPU - preserve tensor's GPU placement
             # This handles multi-GPU scenarios where Accelerate has already placed
             # tensors on the correct GPU for each process
@@ -81,7 +81,7 @@ class DeviceProcessorStep(ProcessorStep):
         else:
             # Either tensor is on CPU, or we're configured for CPU
             # In both cases, use the configured device
-            target_device = self._device
+            target_device = self.tensor_device
 
         # Only move if necessary
         if tensor.device != target_device: