[MXNET-1396][Fit-API] Update default handler logic

[roywei]

Description

Currently the fit api provide user with default event handlers(metric, validation, logging) if none is specified.
It could be troublesome for users if they want to use a different logging handler but have to write the code for metric and validation handler again.
We can provide default handlers one by one, but default handlers use metrics prepared by estimator.prepare_loss_and_metrics(), if user used another set of metrics for user defined handlers (example in test case). The metrics may not be updated correctly. We need to give a warning and add relevant checks if a mix of default and user defined handlers are used.

Changes:

1. provide default handlers one by one
2. check whether default handler and user created handler are using the same set of metrics
3. added unit test, update unit test helper function name
4. further combined nightly test into one runtime function and run under nightly for binaries.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with MXNET-1396 created
• 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:https://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

1. provide default handlers one by one
2. check whether default handler and user created handler are using the same set of metrics
3. added unit test, update unit test helper function name
4. further combined nightly test into one runtime function and run under nightly for binaries.

Comments

• If this change is a backward incompatible change, why must this change be made.
• Interesting edge cases to note here