Fix hpZ with zero element (#5652)
Fix corner cases where hpz secondary partition has zero element. This
ensure that `sec_numel` is at least zero. For this scenario, copying is
really not necessary except that all ranks need to synchronize at the
end of secondary partition. This is a good solution until [2nd tensor
all-gather vs 2nd tensor partition issue
](https://github.com/microsoft/DeepSpeed/blob/master/deepspeed/runtime/zero/partition_parameters.py#L1706)
is properly fixed.
Fixes: #5642
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Co-authored-by: Logan Adams <loadams@microsoft.com>
Co-authored-by: Logan Adams <114770087+loadams@users.noreply.github.com>
