Fix zero reduce bucket size validation (#8266)
## Summary
Reject `zero_optimization.reduce_bucket_size=0` during configuration
validation instead of allowing ZeRO to fail later with a division by
zero during backward.
This changes the field constraint from non-negative to strictly positive
and adds a boundary regression test covering `1` as valid and `0` as
invalid.
Fixes #8260
## Testing
- `python -m pytest tests/unit/runtime/zero/test_zero_config.py -q` (7
passed)
- `pre-commit run --files deepspeed/runtime/zero/config.py
tests/unit/runtime/zero/test_zero_config.py`
Signed-off-by: Cao Yuhang <caoyuhang@fwerkor.com>
Co-authored-by: Olatunji Ruwase <tunji.ruwase@snowflake.com>
