fix: skip compressed allreduce for empty tensors (#7769)
Handle empty buffers in compressed allreduce by early-return and
clearing error buffers to avoid NaNs and needless communication.
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Signed-off-by: T1mn <136770748@qq.com>
Co-authored-by: Masahiro Tanaka <81312776+tohtana@users.noreply.github.com>
Co-authored-by: Olatunji Ruwase <tunji.ruwase@snowflake.com>
