The real inference acceleration is not GPU friendly---batch operator not supported

[zhangzhenyu13]

When inference with the highway early-exit given a batch B;
when |B| = 1, the code is ok to run;
when |B| > 1, the code can corrupt in the code of modeling_highway_bert.py due to
if highway_entropy < self.early_exit_entropy[i]: conditional statement ;
Only one sample can be inferred.
Therefore, the inference is not GPU friendly.