[DistGB] `dgl_partition_to_graphbolt()` crashed

## 🐛 Bug

## To Reproduce

Steps to reproduce the behavior:

```
dgl.distributed.dgl_partition_to_graphbolt(
part_config,
store_eids=True,
graph_formats="coo",
)
```

```
2024-10-03 23:23:24,190 INFO Converting partitions to GraphBolt format
Traceback (most recent call last):
File "/usr/lib/python3.10/runpy.py", line 196, in _run_module_as_main
return _run_code(code, main_globals, None,
File "/usr/lib/python3.10/runpy.py", line 86, in _run_code
exec(code, run_globals)
File "/graphstorm/python/graphstorm/gpartition/convert_to_graphbolt.py", line 72, in
main()
File "/graphstorm/python/graphstorm/gpartition/convert_to_graphbolt.py", line 62, in main
dgl_distributed.dgl_partition_to_graphbolt(
File "/opt/gs-venv/lib/python3.10/site-packages/dgl/distributed/partition.py", line 1991, in dgl_partition_to_graphbolt
part_meta = _convert_partition_to_graphbolt_wrapper(
File "/opt/gs-venv/lib/python3.10/site-packages/dgl/distributed/partition.py", line 1926, in _convert_partition_to_graphbolt_wrapper
rel_path = convert_with_format(part_id=part_id)
File "/opt/gs-venv/lib/python3.10/site-packages/dgl/distributed/partition.py", line 1862, in gb_convert_single_dgl_partition
part = _load_part(part_config, part_id)
File "/opt/gs-venv/lib/python3.10/site-packages/dgl/distributed/partition.py", line 1581, in _load_part
graph, _, _, _, _, _, _ = load_partition(
File "/opt/gs-venv/lib/python3.10/site-packages/dgl/distributed/partition.py", line 342, in load_partition
part_graph_field in part_files
AssertionError: the partition does not contain graph structure: part_graph
```

## Expected behavior

## Environment

- DGL Version (e.g., 1.0): latest nightly build
- Backend Library & Version (e.g., PyTorch 0.4.1, MXNet/Gluon 1.3):
- OS (e.g., Linux):
- How you installed DGL (`conda`, `pip`, source):
- Build command you used (if compiling from source):
- Python version:
- CUDA/cuDNN version (if applicable):
- GPU models and configuration (e.g. V100):
- Any other relevant information:

## Additional context