issues with flag_dynamic and sphere_dynamic dataset

When I tried to run the flag_dynamic case, it has following error:

`ValueError: ragged_rank must be non-negative; got 0.`

And it seems that the dataset is empty, but the size of flag_dynamic dataset folder is more than 30G.

when i run
`ds = meshgraphnets.dataset.load_dataset('meshgraphnets/data/sphere_dynamic', 'train')
ds
`
I can get the output
``