Fix Task Group Mixing

[kurtis-b-1]

Address #15.

Added

• Using task groups in the inner-most loop of MHA runtime. Without it, a long list of BD errors shows up. Checking with previous CI results, this change doesn't seem to negatively affect the performance of MHA.

Changed

• Use same task groups for fill operations as the drain operations in operators such as AXPY, etc.

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[andrej]

Thanks for fixing this so quickly