SSD fix_gamma disabled

[bhheo]

Hi
I found that SSD feature expander uses batchnorm with fixed gamma, because default value is fix_gamma=True.
As I know, there were no batchnorm freeze trick in SSD paper.
So, I fixed code.

[mli]

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[zhreshold]

Thanks for the PR, this might need further investigation whether the change will make the training results better/worse

[mli]

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[mli]

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[mli]

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[mli]

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[mli]

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[mli]

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