Remove warning generated in few examples

[drivanov]

With this fix, the following warnings:

/usr/local/lib/python3.10/dist-packages/torch_geometric/data/in_memory_dataset.py:301: UserWarning: It is not recommended 
to directly access the internal storage format `data` of an 'InMemoryDataset'. If you are absolutely certain what you 
are doing, access the internal storage via `InMemoryDataset._data` instead to suppress this warning. Alternatively, 
you can access stacked individual attributes of every graph via `dataset.{attr_name}`.

will no longer be generated in mem_pool.py, proteins_dmon_pool.py, proteins_mincut_pool.py, seal_link_pred.py examples.