Add node regression in benchmarking code

[Jn-Huang]

As titled.

Provide mse and mae as metrics and loss function.

train.py can run successfully on cora (pretending it is a node regression task).

TODO: update train_minibatch.py and train_gbdt.py

[Jn-Huang]

Update:

1. remove redundant codes

2. fix train_minibatch.py, training is successful

3. fix typo in train.py