[Bug fix] WarmupCosineLR issues (#4688)
Original code missing a `self.` before `warmup_num_steps` so that
`warmup_num_steps` might be 0 and cause math domain error when doing
`math.log(0)`
```py
self.warmup_num_steps = max(2, warmup_num_steps)
self.inverse_log_warm_up = 1.0 / math.log(warmup_num_steps)
```
