[BERT] Pre-trained checkpoint conversion script across frameworks

[nv-rborkar]

As per #317 , we would like all references that use pre-trained weights or start from a checkpoint to provide conversion scripts for use across different frameworks (Pytorch, TF, & other frameworks of choice for submission) to ensure submitters use the same pre-approved checkpoint.

AI Google (reference owner) for BERT
Example script & recipe in #391

[johntran-nv]

@petermattson could you point this to the correct google person?