Add rezero option; register ops for APEX AMP training

[taoleicn]

• add rezero option
• register ops for APEX AMP training

[taoleicn]

Make sure the 3.0.0 (dev) branch has all the changes needed for our model training. I'll test if this branch & version works for ESPNET ASR training.

[hpasapp]

I wonder if we need tests for apex? Otherwise it will break from time to time, and we wont know about it, until someone complains.

[taoleicn]

@hpasapp great point. I'll try adding some tests before next Friday.