Fix device mismatch in TransformerDecoder to enable CPU inference

[jmargeta]

Fixes a device mismatch error (“Expected all tensors to be on the same device”) caused by hard-coded device usage in TransformerDecoder coordinate computations. This update removes the fixed device assignment and enables inference on non-CUDA devices such as CPU.

Exception has occurred: RuntimeError       (note: full exception trace is shown but execution is paused at: _run_module_as_main)
Expected all tensors to be on the same device, but found at least two devices, cuda:0 and cpu!
  File "REDACTED/sam3/sam3/model/decoder.py", line 357, in _get_rpb_matrix
    deltas_y = coords_h.view(1, -1, 1) - boxes_xyxy.reshape(-1, 1, 4)[:, :, 1:4:2]

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