Request to add MobileBert

[msahamed]

Request to add MobileBert

I would like to request to add MobileBert, since it is smaller and faster. The following texts are also a request from huggingface/transformers#4185:

New model addition
This issue is to request adding the MobileBERT model, which was recently released by Carnegie Mellon University, Google Research, and Google Brain.

Model description
MobileBERT is a more computationally-efficient model for achieving BERT-base level accuracy on a smartphone.

Open source status
The model implementation is available: https://github.com/google-research/google-research/tree/master/mobilebert

Model weights are available: uncased_L-24_H-128_B-512_A-4_F-4_OPT (MobileBERT Optimized Uncased English)

Authors: Zhiqing Sun, Hongkun Yu (@saberkun), Xiaodan Song, Renjie Liu, Yiming Yang, Denny Zhou