add fp6 data-type and support sync/async dwordx3 load/store

[eeezio]

Proposed changes

Add packed fp6 data type to support mxfp6-flatmm in the future.
Each thread loads or stores 16 packed 6-bit FP6 data elements at a time, which takes up 12 bytes. Therefore, we also support dwordx3 async/sync load/store operations.

Checklist

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• I have added tests relevant to the introduced functionality, and the unit tests are passing locally
• I have added the test to REGRESSION_TESTS list defined at the top of CMakeLists.txt in tests/CMakeLists.txt, IF the test takes more than 30 seconds to run.
• I have added inline documentation which enables the maintainers with understanding the motivation
• I have removed the stale documentation which is no longer relevant after this pull request
• (If this change is user-facing) I have added release notes which provide the end users with a brief summary of the improvement from this pull request
• I have run clang-format on all changed files
• Any dependent changes have been merged

Discussion

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