Update modeling_tf_bert.py
Author: BruceJust
Pull request: yes

# What does this PR do?
change the attention_mask buid code in modeling_tf_bert.py to support customer attention, such as unilm_mask
former：           
 extended_attention_mask = tf.reshape(
                inputs["attention_mask"], (attention_mask_shape[0], 1, 1, attention_mask_shape[1])
            )
new：
  extended_attention_mask = tf.reshape(
                inputs["attention_mask"], (attention_mask_shape[0], 1, -1, attention_mask_shape[1])
            )
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Fixes # (issue)


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Comments:
this change is very necessary to allowed researcher to build a customized attention, and i foud it has been done in pytorch version.

this change is very necessary to allowed researcher to build a customized attention, and i foud it has been done in pytorch version.