[WIP]Change mshadow shape size to size_t

[ZhennanQin]

Description

(Brief description on what this PR is about)

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• The PR title starts with [MXNET-$JIRA_ID], where $JIRA_ID refers to the relevant JIRA issue created (except PRs with tiny changes)
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage:
• Unit tests are added for small changes to verify correctness (e.g. adding a new operator)
• Nightly tests are added for complicated/long-running ones (e.g. changing distributed kvstore)
• Build tests will be added for build configuration changes (e.g. adding a new build option with NCCL)
• Code is well-documented:
• For user-facing API changes, API doc string has been updated.
• For new C++ functions in header files, their functionalities and arguments are documented.
• For new examples, README.md is added to explain the what the example does, the source of the dataset, expected performance on test set and reference to the original paper if applicable
• Check the API doc at http://mxnet-ci-doc.s3-accelerate.dualstack.amazonaws.com/PR-$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

• Feature1, tests, (and when applicable, API doc)
• Feature2, tests, (and when applicable, API doc)

Comments

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

[ZhennanQin]

@apeforest why mshadow::Shape::size() returns index_t instead of size_t? This will cause trouble when comparing it with other size() returns. Just as this CI failed. Any reason we need negative for this?

[pengzhao-intel]

@zheng-da

[ZhennanQin]

dmlc/mshadow#359 this PR introduces this issue.

[apeforest]

@ZhennanQin In MXNet code, we have many for loop with the type of indexing variable i to be index_t. If the size return type is size_t, we would need to add static cast to all the for loops.

[ZhennanQin]

@apeforest CI can pass after changing back to return size_t. The corresponding MXNet change are shown as this PR. If this solution is OK. I will file separate PR to mshadow.

[apeforest]

LGTM, thanks a lot for your help. Please update the PR title and description when it's ready for review. Thanks

[ZhennanQin]

@apeforest mshadow PR is created at dmlc/mshadow#367, please help to merge it. Then I will edit this PR. Thanks.