Address review feedback for XLA/TPU support
- split_half_float_double: use dtype comparison instead of string-based
type names, without adding on_accelerator filtering that would change
behavior for all backends
- comm.py: clarify that dist_backend fallback is not XLA-specific
- Remove fake TPU device tests per reviewer guidance; XLA accelerator
tests should run on real TPU hardware
Signed-off-by: PKUWZP <zhipeng.rainbowserie@gmail.com>
