Only reminder missing torch_geometric when initializing Raindrop

[WenjieDu]

Issue description

In current version, when importing from pypots in an env without torch_geometric, it will raise missing error, but this is not what we expect. This error should only be raised when initializing necessary models that depend on missing torch_geometric, e.g. Raindrop.

>>> import pypots
2024-05-05 23:00:33 [ERROR]: ❌ No module named 'torch_geometric'
Note torch_geometric is missing, please install it with 'pip install torch_geometric torch_scatter torch_sparse' or 'conda install -c pyg pyg pytorch-scatter pytorch-sparse'
2024-05-05 23:00:33 [ERROR]: ❌ name 'MessagePassing' is not defined
Note torch_geometric is missing, please install it with 'pip install torch_geometric torch_scatter torch_sparse' or 'conda install -c pyg pyg pytorch-scatter pytorch-sparse'
>>>