fix LinearRegressionOutput with empty label

[haohuanw]

Description

There is a behavior regression on LinearRegressionOutput after this CR: 3f3ba92#diff-726d19e2c39201681813130c8e2ac154

The previous CR changes the default TShape::ndim() to -1 instead of 0 but LinearRegressionOutput layer is not updated to handle new behavior of TShape.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• [ x ] Changes are complete (i.e. I finished coding on this PR)
• [ x ] All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• [ x ] Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$PR_ID/$BUILD_ID/index.html
• [ x ] To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• [ x ] cpp-package/examples/test_regress_label.cpp (test whether bind still works when regress_label is not provided to LinearRegressionOutput layer.)

[haohuanw]

@KellenSunderland

[KellenSunderland]

Could use

#include <dmlc/logging.h>

LOG(INFO) << "Running regress label test";

[KellenSunderland]

Same as above (at error level).

[abhinavs95]

@mxnet-label-bot add [operator, backend]