[fix] fix test_zf.py hang bug (#8012)
fix:When running following test command, it may hang.
```pytest -v runtime/zenflow/test_zf.py::TestZenFlowDistributed::test_zenflow_distributed[epoch-1-4-False-0-3]```
The reason is that when param.selected_indices got an empty result, its dtype would be torch.float32 instead of torch.int64. However, if the float32 empty tensor is used as an index just like grad_2d[param.selected_indices, :], it would cause a hang. So in order to solve this bug, I add a dtype cast to int64 when judge the param.selected_indices is empty, which means its original dtype is torch.float32.
Signed-off-by: binchengxiong <binchengxiong@alibaba-inc.com>
Co-authored-by: binchengxiong <binchengxiong@alibaba-inc.com>
Co-authored-by: Masahiro Tanaka <81312776+tohtana@users.noreply.github.com>
