Add transformers to flava requirements (facebookresearch#73)

Summary:
Add HF transformers dependency which was missing

Pull Request resolved: facebookresearch#73

Test Plan:
tested readme on a fresh conda env
python finetune.py config=configs/finetuning/qnli.yaml model.pretrained_model_key=flava_full training.lightning.gpus=0 training.lightning.accelerator=cpu

Reviewed By: katrina433

Differential Revision: D36953232

Pulled By: ankitade

fbshipit-source-id: cee9b0abba15f4d90ff976de882dca0737c41698

examples/flava/requirements.txt

@@ -5,3 +5,4 @@ requests==2.27.1
 DALL-E==0.1
 omegaconf==2.1.2
 hydra-core==1.1.2
+transformers==4.16.0