Generate text from inputs_embeds for seq2seq models like BART or T5
# 🚀 Feature request

Generate text from inputs_embeds for seq2seq models like BART or T5

## Motivation

Many advanced researches now rely on generation of text from an embedding vector. Currently we can perform forward pass through BART or T5 by just inputing inputs_embeds but this is not possible in generate function.

At the moment I have to go around this issue by calling the encoder to get encoder_outputs, and used it as  an input to the generation function directly:
```python
 encoder_outputs = model.base_model.encoder(inputs_embeds=input_embeds, return_dict=True)
 out = model.generate(encoder_outputs=encoder_outputs)
```
Even this does not show any error message but I am not sure this is the right thing to do, any comment? Can you please support this function as did with the forward function?

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