Use BioGptModel to get features

We can't use `BioGptForCausalLM` to get the features as it returns logits for next token prediction. I'm pretty sure we want `BioGptModel` to give us last_hidden_state

README.md

@@ -30,9 +30,9 @@ set a seed for reproducibility:
 Here is how to use this model to get the features of a given text in PyTorch:
 
 ```python
-from transformers import BioGptTokenizer, BioGptForCausalLM
+from transformers import BioGptTokenizer, BioGptModel
 tokenizer = BioGptTokenizer.from_pretrained("microsoft/biogpt")
-model = BioGptForCausalLM.from_pretrained("microsoft/biogpt")
+model = BioGptModel.from_pretrained("microsoft/biogpt")
 text = "Replace me by any text you'd like."
 encoded_input = tokenizer(text, return_tensors='pt')
 output = model(**encoded_input)