Address tohtana's review feedback for XLA/TPU support
- _normalize_device_index: handle str and torch.device types in addition
to int, since callers pass LOCAL_RANK strings and torch.device objects
- real_accelerator: catch RuntimeError from get_xla_supported_devices()
when torch_xla is installed but no TPU/PJRT runtime is available
Signed-off-by: PKUWZP <zhipeng.rainbowserie@gmail.com>
