FP16 training fix

[vince62s]

Here is the issue:

For FP16 training, we use Nvidia/apex.
They changed the API to something called AMP and we adapted to this new API on June 13.

However, a few things are broken in this new API when it comes to handle FusedAdam.

On Aug 8 they included FusedAdam in the AMP new API but I realized that:

O2 level does not work at all for both Adam and FuseAdam (O2 was our Default level).
see NVIDIA/apex#475
O1 level works fine for Adam but does work (unstable) with FusedAdam.

This PR will:

1. Default to O1 with Adam/AMP (FP16 training only)
2. Use the Legacy FusedAdam code that we have included in optimizer.py

This solution should be temporary since Nvidia works on including AMP directly in Pytorch.

As of this PR, Accuracy/PPL are ok for Adam FP32, Adam FP16, FusedAdam FP16