add model card for t5-base-squad (#4409)

* add model card for t5-base-squad

* Update model_cards/valhalla/t5-base-squad/README.md
Co-authored-by: Julien Chaumond <chaumond@gmail.com>

model_cards/valhalla/t5-base-squad/README.md

+# T5 for question-answering
+This is T5-base model fine-tuned on SQuAD1.1 for QA using text-to-text approach
+
+## Model training
+This model was trained on colab TPU with 35GB RAM for 4 epochs
+
+## Results:
+| Metric      | #Value  |
+|-------------|---------|
+| Exact Match | 81.5610 |
+| F1          | 89.9601 |
+
+## Model in Action 🚀
+```
+from transformers import AutoModelWithLMHead, AutoTokenizer
+
+tokenizer = AutoTokenizer.from_pretrained("valhalla/t5-base-squad")
+model = AutoModelWithLMHead.from_pretrained("valhalla/t5-base-squad")
+
+def get_answer(question, context):
+  input_text = "question: %s  context: %s </s>" % (question, context)
+  features = tokenizer.batch_encode_plus([input_text], return_tensors='pt')
+
+  out = model.generate(input_ids=features['input_ids'], 
+               attention_mask=features['attention_mask'])
+  
+  return tokenizer.decode(out[0])
+
+context = "In Norse mythology, Valhalla is a majestic, enormous hall located in Asgard, ruled over by the god Odin."
+question = "What is Valhalla ?"
+
+get_answer(question, context)
+# output: 'a majestic, enormous hall located in Asgard, ruled over by the god Odin'
+```
+Play with this model [![Open In Colab](https://colab.research.google.com/assets/colab-badge.svg)](https://colab.research.google.com/drive/1a5xpJiUjZybfU9Mi-aDkOp116PZ9-wni?usp=sharing)
+
+> Created by Suraj Patil [![Github icon](https://cdn0.iconfinder.com/data/icons/octicons/1024/mark-github-32.png)](https://github.com/patil-suraj/)
+[![Twitter icon](https://cdn0.iconfinder.com/data/icons/shift-logotypes/32/Twitter-32.png)](https://twitter.com/psuraj28)