Fix optimizer to work with horovod

pytorch_pretrained_bert/optimization.py

@@ -17,6 +17,7 @@
 import math
 import torch
 from torch.optim import Optimizer
+from torch.optim.optimizer import required
 from torch.nn.utils import clip_grad_norm_
 
 def warmup_cosine(x, warmup=0.002):
@@ -55,10 +56,10 @@ class BertAdam(Optimizer):
         weight_decay_rate: Weight decay. Default: 0.01
         max_grad_norm: Maximum norm for the gradients (-1 means no clipping). Default: 1.0
     """
-    def __init__(self, params, lr, warmup=-1, t_total=-1, schedule='warmup_linear',
+    def __init__(self, params, lr=required, warmup=-1, t_total=-1, schedule='warmup_linear',
                  b1=0.9, b2=0.999, e=1e-6, weight_decay_rate=0.01,
                  max_grad_norm=1.0):
-        if not lr >= 0.0:
+        if lr is not required and lr < 0.0:
             raise ValueError("Invalid learning rate: {} - should be >= 0.0".format(lr))
         if schedule not in SCHEDULES:
             raise ValueError("Invalid schedule parameter: {}".format(schedule))