do not pin memory when device is CPU to avoid warning

[pedrormjunior]

This seems to fix issue #854.

Without this modification, it raises the following warning:

/path/to/.venv/lib/python3.13/site-packages/torch/utils/data/dataloader.py:665: UserWarning: 'pin_memory' argument is set as true but no accelerator is found, then device pinned memory won't be used.
  warnings.warn(warn_msg)

I did not think about collateral effects of this change, other use cases, etc. Please, check.