Skip zero-sized parameters in HP fragment mapping (#8298)
## Summary
- exclude zero-sized parameters from ZeRO-1/2 HP-fragment overlap
mapping
- avoid constructing an empty fragment when an empty parameter's offset
lies inside a partition
Fixes #8297.
## Testing
- current `master` (`edaa722`), 2x A6000: minimized ZeRO-1 reproducer
fails before the change with `fragment start 8 should be < fragment_end
8` and completes after the change
- 2-process CPU/Gloo: minimized reproducer completes with both ZeRO-1
and ZeRO-2 after the change
- the trailing zero-sized-parameter reproducer from #8279 still
completes
- `python3 -m py_compile deepspeed/utils/mixed_precision_linkage.py`
Signed-off-by: Cao Yuhang <caoyuhang@fwerkor.com>
Co-authored-by: Olatunji Ruwase <tunji.ruwase@snowflake.com>
