feat(DeviceProcessor): Enhance tensor processing with device detection and float dtype conversion

- Improved the _process_tensor method to preserve GPU placement for tensors already on a GPU, facilitating multi-GPU training scenarios.
- Introduced a new _detect_device method in TokenizerProcessor to ensure tokenized tensors match the device of existing tensors in transitions.
- Added comprehensive unit tests to validate the functionality of device detection and float dtype conversion across various scenarios.

src/lerobot/processor/device_processor.py

@@ -66,9 +66,26 @@ class DeviceProcessor:
             self._target_float_dtype = None
 
     def _process_tensor(self, tensor: torch.Tensor) -> torch.Tensor:
-        """Process a tensor by moving to device and optionally converting float dtype."""
-        # Move to device first
-        tensor = tensor.to(self.device, non_blocking=self.non_blocking)
+        """Process a tensor by moving to device and optionally converting float dtype.
+
+        If the tensor is already on a GPU and we're configured for a GPU, it preserves
+        that GPU placement (useful for multi-GPU training with Accelerate).
+        Otherwise, it moves to the configured device.
+        """
+        # Determine target device
+        if tensor.is_cuda and self._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
+            target_device = tensor.device
+        else:
+            # Either tensor is on CPU, or we're configured for CPU
+            # In both cases, use the configured device
+            target_device = self._device
+
+        # Only move if necessary
+        if tensor.device != target_device:
+            tensor = tensor.to(target_device, non_blocking=self.non_blocking)
 
         # Convert float dtype if specified and tensor is floating point
         if self._target_float_dtype is not None and tensor.is_floating_point():