Create README.md for xlnet_large_squad (#2942)

model_cards/ahotrod/xlnet_large_squad2_512/README.md

+## XLNet large language model fine-tuned on SQuAD2.0
+
+### with the following results:
+
+```
+  "exact": 82.07698138633876,
+  "f1": 85.898874470488,
+  "total": 11873,
+  "HasAns_exact": 79.60526315789474,
+  "HasAns_f1": 87.26000954590184,
+  "HasAns_total": 5928,
+  "NoAns_exact": 84.54163162321278,
+  "NoAns_f1": 84.54163162321278,
+  "NoAns_total": 5945,
+  "best_exact": 83.22243746315169,
+  "best_exact_thresh": -11.112004280090332,
+  "best_f1": 86.88541353813282,
+  "best_f1_thresh": -11.112004280090332
+```
+### from script:
+```
+python -m torch.distributed.launch --nproc_per_node=2 ${RUN_SQUAD_DIR}/run_squad.py \
+  --model_type xlnet \
+  --model_name_or_path xlnet-large-cased \
+  --do_train \
+  --train_file ${SQUAD_DIR}/train-v2.0.json \
+  --predict_file ${SQUAD_DIR}/dev-v2.0.json \
+  --version_2_with_negative \
+  --num_train_epochs 3 \
+  --learning_rate 3e-5 \
+  --adam_epsilon 1e-6 \
+  --max_seq_length 512 \
+  --doc_stride 128 \
+  --save_steps 2000 \
+  --per_gpu_train_batch_size 1 \
+  --gradient_accumulation_steps 24 \
+  --output_dir ${MODEL_PATH}
+
+CUDA_VISIBLE_DEVICES=0 python ${RUN_SQUAD_DIR}/run_squad_II.py \
+  --model_type xlnet \
+  --model_name_or_path ${MODEL_PATH} \
+  --do_eval \
+  --train_file ${SQUAD_DIR}/train-v2.0.json \
+  --predict_file ${SQUAD_DIR}/dev-v2.0.json \
+  --version_2_with_negative \
+  --max_seq_length 512 \
+  --per_gpu_eval_batch_size 48 \
+  --output_dir ${MODEL_PATH}
+```
+### using the following system & software:
+```
+OS/Platform: Linux-4.15.0-76-generic-x86_64-with-debian-buster-sid
+GPU/CPU: 2 x NVIDIA 1080Ti / Intel i7-8700
+Transformers: 2.1.1
+PyTorch: 1.4.0
+TensorFlow: 2.1.0
+Python: 3.7.6
+```
+### Inferencing / prediction works with Transformers v2.4.1, the latest version tested
+
+### Utilize this xlnet_large_squad2_512 fine-tuned model with:
+```python
+config_class, model_class, tokenizer_class = \
+        XLNetConfig, XLNetforQuestionAnswering, XLNetTokenizer
+model_name_or_path = "ahotrod/xlnet_large_squad2_512"
+config = config_class.from_pretrained(model_name_or_path)
+tokenizer = tokenizer_class.from_pretrained(model_name_or_path, do_lower_case=True)
+model = model_class.from_pretrained(model_name_or_path, config=config)
+```
+### or the AutoModels (AutoConfig, AutoTokenizer & AutoModel) should also work, however I have yet to use them in my apps & confirm:
+```python
+from transformers import AutoConfig, AutoTokenizer, AutoModel
+model_name_or_path = "ahotrod/xlnet_large_squad2_512"
+config = AutoConfig.from_pretrained(model_name_or_path)
+tokenizer = AutoTokenizer.from_pretrained(model_name_or_path, do_lower_case=True)
+model = AutoModel.from_pretrained(model_name_or_path, config=config)
+```