[WarmupDecayLR] fix log(0) & 1/log(1) bugs (#772)

* fix log(0) & 1/log(1) bugs

* simplify
Co-authored-by: Jeff Rasley <jerasley@microsoft.com>
Co-authored-by: Reza Yazdani <44502768+RezaYazdaniAminabadi@users.noreply.github.com>
Co-authored-by: Cheng Li <pistasable@gmail.com>

deepspeed/runtime/lr_schedules.py

@@ -706,8 +706,8 @@ class WarmupLR(object):
         self.min_lrs = self._format_param(self.optimizer, warmup_min_lr, "min_lr")
         self.max_lrs = self._format_param(self.optimizer, warmup_max_lr, "max_lr")
         self.delta_lrs = [big - small for big, small in zip(self.max_lrs, self.min_lrs)]
-        self.warmup_num_steps = warmup_num_steps
-        self.inverse_log_warm_up = 1.0 / math.log(warmup_num_steps)
+        self.warmup_num_steps = max(2, warmup_num_steps)
+        self.inverse_log_warm_up = 1.0 / math.log(self.warmup_num_steps)
         self.last_batch_iteration = last_batch_iteration
 
     def get_lr(self):