Update pretrain_bert.py (deepspeedai#355)

fix a bug that cause
File "pretrain_bert.py", line 91, in loss_func
    lm_loss_, sop_logits = output_tensor
    ^^^^^^^^^^^^^^^^^^^^
ValueError: too many values to unpack (expected 2)

Author: lzzmm

pretrain_bert.py

@@ -87,7 +87,7 @@ def data_post_process(data, data_sampler_state_dict):
     return data
 
 def loss_func(loss_mask, sentence_order, output_tensor):
-    lm_loss_, sop_logits = output_tensor
+    lm_loss_, sop_logits, _ = output_tensor
 
     lm_loss_ = lm_loss_.float()
     loss_mask = loss_mask.float()