* Remove dead code

params_already_reduced is not used

* Prevent evaluation of debug strings

Debug strings are evaluated even when logging is disabled

* Use contiguous gradients tensor reduce scatter between ranks

Use allreduce instead of reduce scatter. lower cpu overhead.

* move overflow tracker to optimizer.step

Don't check overflow in gradients for every bucket.
Do overflow chack once on grad flat buffer just before optimizer step

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Co-authored-by: NOlatunji Ruwase <olruwase@microsoft.com>