Add async grad allreduce to GPT training with O2 mixed precision

[ericharper]

What does this PR do ?

When adding pipeline parallelism implementation we had to disable async allreduce in the case of tensor parallelism only. This causes a perf regression when training small models. With this PR we are able to add back async grad allreduce for O2 training.

A subsequent PR will implement async grad allreduce for O1 training.

Collection: NLP

Changelog

• Update training_step with async handler from O2 recipe

Usage

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Additional Information

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[ericharper]

Closed in favor of #4084