Fix test_tsagcn

[cshjin]

Fix the issue in #186
Changes

• send edge_index to device
• assign self_mat to device to match edge_index
• passed the pytest

$ pytest test/
============================================ test session starts =============================================
platform linux -- Python 3.7.16, pytest-7.1.2, pluggy-1.0.0
rootdir: /home/***/pytorch_geometric_temporal
collected 93 items                                                                                           

test/attention_test.py ........                                                                        [  8%]
test/batch_test.py .............................                                                       [ 39%]
test/dataset_test.py ..........................................                                        [ 84%]
test/heterogeneous_test.py .                                                                           [ 86%]
test/recurrent_test.py .............                                                                   [100%]

======================================= 93 passed in 119.87s (0:01:59) =======================================