Allow Users to Specify Data Type (is_multilabel) in Config File

[donglihe-hub]

What does this PR do?

This is a follow-up for #337.

• The task type can either be detected automatically (the default behavior currently) or be specified by users (Li-Chung will explain the motivation).

This change is backward compatible.

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

Test APIs used by main.py.

• Test Pass
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• 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)
• 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.