Update README of QA examples (huggingface#14172)

examples/legacy/question-answering/README.md

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+#### Fine-tuning BERT on SQuAD1.0 with relative position embeddings
+
+The following examples show how to fine-tune BERT models with different relative position embeddings. The BERT model 
+`bert-base-uncased` was pretrained with default absolute position embeddings. We provide the following pretrained 
+models which were pre-trained on the same training data (BooksCorpus and English Wikipedia) as in the BERT model 
+training, but with different relative position embeddings. 
+
+* `zhiheng-huang/bert-base-uncased-embedding-relative-key`, trained from scratch with relative embedding proposed by 
+Shaw et al., [Self-Attention with Relative Position Representations](https://arxiv.org/abs/1803.02155)
+* `zhiheng-huang/bert-base-uncased-embedding-relative-key-query`, trained from scratch with relative embedding method 4 
+in Huang et al. [Improve Transformer Models with Better Relative Position Embeddings](https://arxiv.org/abs/2009.13658)
+* `zhiheng-huang/bert-large-uncased-whole-word-masking-embedding-relative-key-query`, fine-tuned from model 
+`bert-large-uncased-whole-word-masking` with 3 additional epochs with relative embedding method 4 in Huang et al. 
+[Improve Transformer Models with Better Relative Position Embeddings](https://arxiv.org/abs/2009.13658)
+
+
+##### Base models fine-tuning
+
+```bash
+export CUDA_VISIBLE_DEVICES=0,1,2,3,4,5,6,7
+python -m torch.distributed.launch --nproc_per_node=8 ./examples/question-answering/run_squad.py \
+    --model_name_or_path zhiheng-huang/bert-base-uncased-embedding-relative-key-query \
+    --dataset_name squad \
+    --do_train \
+    --do_eval \
+    --learning_rate 3e-5 \
+    --num_train_epochs 2 \
+    --max_seq_length 512 \
+    --doc_stride 128 \
+    --output_dir relative_squad \
+    --per_device_eval_batch_size=60 \
+    --per_device_train_batch_size=6
+```
+Training with the above command leads to the following results. It boosts the BERT default from f1 score of 88.52 to 90.54.
+
+```bash
+'exact': 83.6802270577105, 'f1': 90.54772098174814
+```
+
+The change of `max_seq_length` from 512 to 384 in the above command leads to the f1 score of 90.34. Replacing the above 
+model `zhiheng-huang/bert-base-uncased-embedding-relative-key-query` with 
+`zhiheng-huang/bert-base-uncased-embedding-relative-key` leads to the f1 score of 89.51. The changing of 8 gpus to one 
+gpu training leads to the f1 score of 90.71.
+
+##### Large models fine-tuning
+
+```bash
+export CUDA_VISIBLE_DEVICES=0,1,2,3,4,5,6,7
+python -m torch.distributed.launch --nproc_per_node=8 ./examples/question-answering/run_squad.py \
+    --model_name_or_path zhiheng-huang/bert-large-uncased-whole-word-masking-embedding-relative-key-query \
+    --dataset_name squad \
+    --do_train \
+    --do_eval \
+    --learning_rate 3e-5 \
+    --num_train_epochs 2 \
+    --max_seq_length 512 \
+    --doc_stride 128 \
+    --output_dir relative_squad \
+    --per_gpu_eval_batch_size=6 \
+    --per_gpu_train_batch_size=2 \
+    --gradient_accumulation_steps 3
+```
+Training with the above command leads to the f1 score of 93.52, which is slightly better than the f1 score of 93.15 for 
+`bert-large-uncased-whole-word-masking`.
+
+#### Distributed training
+
+Here is an example using distributed training on 8 V100 GPUs and Bert Whole Word Masking uncased model to reach a F1 > 93 on SQuAD1.1:
+
+```bash
+python -m torch.distributed.launch --nproc_per_node=8 ./examples/question-answering/run_squad.py \
+    --model_name_or_path bert-large-uncased-whole-word-masking \
+    --dataset_name squad \
+    --do_train \
+    --do_eval \
+    --learning_rate 3e-5 \
+    --num_train_epochs 2 \
+    --max_seq_length 384 \
+    --doc_stride 128 \
+    --output_dir ./examples/models/wwm_uncased_finetuned_squad/ \
+    --per_device_eval_batch_size=3   \
+    --per_device_train_batch_size=3   \
+```
+
+Training with the previously defined hyper-parameters yields the following results:
+
+```bash
+f1 = 93.15
+exact_match = 86.91
+```
+
+This fine-tuned model is available as a checkpoint under the reference
+[`bert-large-uncased-whole-word-masking-finetuned-squad`](https://huggingface.co/bert-large-uncased-whole-word-masking-finetuned-squad).
+
+## Results
+
+Larger batch size may improve the performance while costing more memory.
+
+##### Results for SQuAD1.0 with the previously defined hyper-parameters:
+
+```python
+{
+"exact": 85.45884578997162,
+"f1": 92.5974600601065,
+"total": 10570,
+"HasAns_exact": 85.45884578997162,
+"HasAns_f1": 92.59746006010651,
+"HasAns_total": 10570
+}
+```
+
+##### Results for SQuAD2.0 with the previously defined hyper-parameters:
+
+```python
+{
+"exact": 80.4177545691906,
+"f1": 84.07154997729623,
+"total": 11873,
+"HasAns_exact": 76.73751686909581,
+"HasAns_f1": 84.05558584352873,
+"HasAns_total": 5928,
+"NoAns_exact": 84.0874684608915,
+"NoAns_f1": 84.0874684608915,
+"NoAns_total": 5945
+}
+```