[PERF] Improve add_bias kernel for small bias length

[ptrendx]

Description

This PR improves performance of the add_bias kernel for FullyConnected layer when the bias length is small, by computing multiple rows of the result in a single thread block.

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

[mxnet-bot]

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