Add model card for Google AI's BERT Miniatures (#3301)

This model card is intended to be shared among all models under google/bert_uncased_*
(We'll need some support from HuggingFace to get this card cross-linked from all models)

model_cards/iuliaturc/bert_uncased_L-2_H-128_A-2/README.md

+BERT Miniatures
+===
+
+This is the set of 24 BERT models referenced in [Well-Read Students Learn Better: On the Importance of Pre-training Compact Models](https://arxiv.org/abs/1908.08962) (English only, uncased, trained with WordPiece masking).
+
+We have shown that the standard BERT recipe (including model architecture and training objective) is effective on a wide range of model sizes, beyond BERT-Base and BERT-Large. The smaller BERT models are intended for environments with restricted computational resources. They can be fine-tuned in the same manner as the original BERT models. However, they are most effective in the context of knowledge distillation, where the fine-tuning labels are produced by a larger and more accurate teacher.
+
+Our goal is to enable research in institutions with fewer computational resources and encourage the community to seek directions of innovation alternative to increasing model capacity.
+
+You can download the 24 BERT miniatures either from the [official BERT Github page](https://github.com/google-research/bert/), or via HuggingFace from the links below:
+
+|   |H=128|H=256|H=512|H=768|
+|---|:---:|:---:|:---:|:---:|
+| **L=2**  |[**2/128 (BERT-Tiny)**][2_128]|[2/256][2_256]|[2/512][2_512]|[2/768][2_768]|
+| **L=4**  |[4/128][4_128]|[**4/256 (BERT-Mini)**][4_256]|[**4/512 (BERT-Small)**][4_512]|[4/768][4_768]|
+| **L=6**  |[6/128][6_128]|[6/256][6_256]|[6/512][6_512]|[6/768][6_768]|
+| **L=8**  |[8/128][8_128]|[8/256][8_256]|[**8/512 (BERT-Medium)**][8_512]|[8/768][8_768]|
+| **L=10** |[10/128][10_128]|[10/256][10_256]|[10/512][10_512]|[10/768][10_768]|
+| **L=12** |[12/128][12_128]|[12/256][12_256]|[12/512][12_512]|[**12/768 (BERT-Base)**][12_768]|
+
+Note that the BERT-Base model in this release is included for completeness only; it was re-trained under the same regime as the original model.
+
+Here are the corresponding GLUE scores on the test set:
+
+|Model|Score|CoLA|SST-2|MRPC|STS-B|QQP|MNLI-m|MNLI-mm|QNLI(v2)|RTE|WNLI|AX|
+|---|:---:|:---:|:---:|:---:|:---:|:---:|:---:|:---:|:---:|:---:|:---:|:---:|
+|BERT-Tiny|64.2|0.0|83.2|81.1/71.1|74.3/73.6|62.2/83.4|70.2|70.3|81.5|57.2|62.3|21.0|
+|BERT-Mini|65.8|0.0|85.9|81.1/71.8|75.4/73.3|66.4/86.2|74.8|74.3|84.1|57.9|62.3|26.1|
+|BERT-Small|71.2|27.8|89.7|83.4/76.2|78.8/77.0|68.1/87.0|77.6|77.0|86.4|61.8|62.3|28.6|
+|BERT-Medium|73.5|38.0|89.6|86.6/81.6|80.4/78.4|69.6/87.9|80.0|79.1|87.7|62.2|62.3|30.5|
+
+For each task, we selected the best fine-tuning hyperparameters from the lists below, and trained for 4 epochs:
+- batch sizes: 8, 16, 32, 64, 128
+- learning rates: 3e-4, 1e-4, 5e-5, 3e-5
+
+If you use these models, please cite the following paper:
+
+```
+@article{turc2019,
+  title={Well-Read Students Learn Better: On the Importance of Pre-training Compact Models},
+  author={Turc, Iulia and Chang, Ming-Wei and Lee, Kenton and Toutanova, Kristina},
+  journal={arXiv preprint arXiv:1908.08962v2 },
+  year={2019}
+}
+```
+
+[2_128]: https://huggingface.co/google/bert_uncased_L-2_H-128_A-2
+[2_256]: https://huggingface.co/google/bert_uncased_L-2_H-256_A-4
+[2_512]: https://huggingface.co/google/bert_uncased_L-2_H-512_A-8
+[2_768]: https://huggingface.co/google/bert_uncased_L-2_H-768_A-12
+[4_128]: https://huggingface.co/google/bert_uncased_L-4_H-128_A-2
+[4_256]: https://huggingface.co/google/bert_uncased_L-4_H-256_A-4
+[4_512]: https://huggingface.co/google/bert_uncased_L-4_H-512_A-8
+[4_768]: https://huggingface.co/google/bert_uncased_L-4_H-768_A-12
+[6_128]: https://huggingface.co/google/bert_uncased_L-6_H-128_A-2
+[6_256]: https://huggingface.co/google/bert_uncased_L-6_H-256_A-4
+[6_512]: https://huggingface.co/google/bert_uncased_L-6_H-512_A-8
+[6_768]: https://huggingface.co/google/bert_uncased_L-6_H-768_A-12
+[8_128]: https://huggingface.co/google/bert_uncased_L-8_H-128_A-2
+[8_256]: https://huggingface.co/google/bert_uncased_L-8_H-256_A-4
+[8_512]: https://huggingface.co/google/bert_uncased_L-8_H-512_A-8
+[8_768]: https://huggingface.co/google/bert_uncased_L-8_H-768_A-12
+[10_128]: https://huggingface.co/google/bert_uncased_L-10_H-128_A-2
+[10_256]: https://huggingface.co/google/bert_uncased_L-10_H-256_A-4
+[10_512]: https://huggingface.co/google/bert_uncased_L-10_H-512_A-8
+[10_768]: https://huggingface.co/google/bert_uncased_L-10_H-768_A-12
+[12_128]: https://huggingface.co/google/bert_uncased_L-12_H-128_A-2
+[12_256]: https://huggingface.co/google/bert_uncased_L-12_H-256_A-4
+[12_512]: https://huggingface.co/google/bert_uncased_L-12_H-512_A-8
+[12_768]: https://huggingface.co/google/bert_uncased_L-12_H-768_A-12