Fix precision vars initialization in TRT

[Kh4L]

Description

Fix precision vars initialization in TRT

[mxnet-bot]

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[DickJC123]

Yes, one needs to remember that data members like fp16_mode_ and int8_mode_ are not initialized by default. I see that calibration_iters_ is initted only when needed (i.e. when int8_mode_ is true). LGTM.