Fix issues for `multiclass`

[Eleven1Liu]

What does this PR do?

1. torch_trainer.py: Remove config.multiclass, and use model.multiclass instead.
2. nn/metrics.py: Check P/R/RP/nDCG@k (k > 1) in multiclass mode.
3. nn/data_utils.py: No train_test_split when training data is not available.

Test CLI & API (bash tests/autotest.sh)

Test APIs used by main.py.

• [x] Test Pass
  • (Copy and paste the last outputted line here.)
• [ ] Not Applicable (i.e., the PR does not include API changes.)

Check API Document

If any new APIs are added, please check if the description of the APIs is added to API document.

• [ ] API document is updated (linear, nn)
• [x] Not Applicable (i.e., the PR does not include API changes.)

Test quickstart & API (bash tests/docs/test_changed_document.sh)

If any APIs in quickstarts or tutorials are modified, please run this test to check if the current examples can run correctly after the modified APIs are released.