[v1.x][BUGFIX] Export attributes of transformer operators

[PawelGlomski-Intel]

Description

sg_mkldnn_selfatt_qk and sg_mkldnn_selfatt_valatt ops, when not quantized, are not exporting their attributes to the *-symbol.json file, thus making the SymbolBlock.imports/model.load_checkpoint functions crash with an error: "mxnet.base.MXNetError: MXNetError: Required parameter heads of int is not presented, in operator sg_mkldnn_selfatt_*(name="sg_mkldnn_selfatt_*")" when trying to load a model that uses them.

This change fixes this issue.

Checklist

Essentials

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

Changes

• Assign attributes of transformer operators

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[PawelGlomski-Intel]

@bgawrych Can you please review this?

[PawelGlomski-Intel]

@akarbown I believe this can be merged