[FEATURE] Add query_keys transformer version without split

[agrabows]

Description

MXNet is fusing split, reshape, swapaxis and batch_dot operators for performance purpose. In gpt-2 model this fuse could be done as well if we exclude split. ->

Checklist

Essentials

• [x] PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• [x] Changes are complete (i.e. I finished coding on this PR)
• [x] All changes have test coverage
• [x] Code is well-documented

[mxnet-bot]

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[agrabows]

[agrabows]

@mxnet-bot run ci [unix-gpu]

[mxnet-bot]

Jenkins CI successfully triggered : [unix-gpu]