Python BucketingModule bind() with grad_req = 'add'

[slyforce]

Description

This pull request adds the possibility of changing the grad_req of BucketingModule to values other than the default 'write' in the Python interface.

Previously, grad_req would default to 'write' every time the bucket is switched causing gradient accumulation not to work whenever a non-default bucket key was used.

Also a new unit-test case has been added to test 1. the correct binding of sub-modules and 2. gradient accumulation with different grad_req values.

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

• BucketingModule now remembers grad_req from bind() call. ,
• Unit-test for different values of grad_req for BucketingModule.

[sandeep-krishnamurthy]

Thanks. LGTM.

@szha - Can you please take a review at this PR?