Expose Consecutive Hysteresis to Users

[Quentin-Anthony]

There's already a nice consecutive_hysteresis feature in the DynamicLossScaler that replenishes the hysteresis whenever a non-overflowing iteration is encountered. This is useful for training runs that periodically face instabilities or bad samples.

This PR exposes this existing feature to users via a new config option consecutive_hysteresis, which is added to the fp16 dict like:

"fp16": {
    "enabled": true,
    "loss_scale": 0,
    "loss_scale_window": 1000,
    "hysteresis": 2,
    "consecutive_hysteresis": true,
    "min_loss_scale": 1
}

So if "consecutive_hysteresis": true, "hysteresis: 2", and every 2k iterations (outside the loss window) there's a single loss overflow, we'd restore the hysteresis to 2 after each overflow and won't update the loss scale. Alternatively, if "consecutive_hysteresis": false, we'd update the loss scale after every hysteresis=2 loss overflows.