avoid test relu at the origin due to discontinuous gradient

[ElaineBao]

Description

For relu, if an input is very close to the origin, numeric gradient cannot be compute due to discontinuous gradient at the origin. In order to avoid this test case, we replace small problematic inputs with 1.0.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• Changes are complete (i.e. I finished coding on this PR)
• 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)
• 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
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Fix test case of MKLDNN relu

[pengzhao-intel]

LGTM, thanks for the fix.

[wkcn]

LGTM. Thank you!

[wkcn]

Merged. Thank you : )