change tf.math.divide with int(/) in distilbert model
Author: yis11178
Pull request: yes


# What does this PR do?

This PR changes `tf.math.divide` with `int( / )` for variable `dim_per_head`. Originally `dim_per_head` is returned as a `float64` tensor and is included in the TF graph. This is not supported by some machines and there is not a solid reason to include it into the TF graph. Hence, this PR removes it from the TF graph and allows for better compatibility with user's device. 
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Comments:
This is a simple change with a clear rationale, and all tests pass, so I'm happy to merge it. Thank you for this PR!