Add `inputs_embeds` functionality when generating with BioGPT 
# What does this PR do?


This PR extends https://github.com/huggingface/transformers/pull/21405 by @gante to BioGPT, making it accept inputs_embeds when generating.

```
import torch
from transformers import  BioGptTokenizer, BioGptForCausalLM


model = BioGptForCausalLM.from_pretrained("microsoft/biogpt")
tokenizer = BioGptTokenizer.from_pretrained("microsoft/biogpt")

inputs_embeds = torch.rand((1, 1, 1024)) # 1 dummy soft-prompt token embeddings
attention_mask = torch.ones(inputs_embeds.shape[:2], dtype=torch.long)
filler_input_ids = torch.LongTensor([[model.config.bos_token_id]])

model.generate(filler_input_ids,attention_mask = attention_mask, inputs_embeds=inputs_embeds, max_new_tokens=300, num_beams=4)
```
# Who can Review?
@gante 

_The documentation is not available anymore as the PR was closed or merged._<|||||>review request: @gante <|||||>Thanks for your contribution!