Fix ZeRO-1/2 with zero-sized parameters (#8280)
## Summary
- skip zero-sized parameters before ZeRO-1/2 gradient reduction
- add regression coverage for stages 1 and 2
## Testing
- `pytest -q tests/unit/runtime/zero/test_zero_empty_param.py -s`
- original #8279 reproducer on A6000 with PyTorch 2.13.0+cu126 for
ZeRO-1 and ZeRO-2
- `pre-commit run --files deepspeed/runtime/zero/stage_1_and_2.py
tests/unit/runtime/zero/test_zero_empty_param.py`
Fixes #8279
Signed-off-by: Cao Yuhang <caoyuhang@fwerkor.com>
