Fix bug in resuming-from-checkpoint not working with mixed precision training

[m43]

Without the commit, resuming training from a checkpoint would induce a disruption in loss/accuracy curves, as is the case for the orange line from step 300. With this commit, the state_dict for the gradient scaler would also be saved into the checkpoint. This gives the dark blue curve that correctly resume from where the previous training left off. This only affects training and checkpoints that used mixed precision.

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