[EncoderDecoder] Make sure `use_cache` is set to `True` for all Bert2Bert, Roberta2Roberta by default
At the moment when one loads a Bert2Bert:

```python
model = EncoderDecoderModel.from_encoder_decoder_pretrained("bert-base-cased", "bert-base-cased")
```

does not automatically set `use_cache` to True -> so that the user "silently" has to be fine with a much slower than optimal inference speed. Also all Bert2Bert configs online don't have `use_cache` set to True. This should be changed at least for the heavily used Bert2Bert models.

I'll try to take care of that in the next couple days. Also pinging @patil-suraj for information. Thanks @Narsil for binging up the topic. 
@patrickvonplaten 

Can we instead set `use_cache` to `True` by default in `generate`? That way we won't need to rely on `config` 

Right now, the `generate` [docstring](https://github.com/huggingface/transformers/blob/master/src/transformers/generation_utils.py#L689) says that it defaults to `True`, but it's set to `None`

https://github.com/huggingface/transformers/blob/28d74872cc049e0cbee3fafd15cbbabfe348ebd4/src/transformers/generation_utils.py#L618
<|||||>Hmm, that goes a bit against the philosophy because we never "set" any variables in `generate()`. We should do it in `EncoderDecoderConfig` and in `from_encoder_decoder_pretrained`. Note that all args in `generate()` are set to `None`, but default to the respective config defaults which should be set correctly<|||||>Also `use_cache` is newly introduced in bert/roberta config and is `True` by default, so even if the model's config file online doesn't have  `use_cache` it should still be `True,` no?

Could you maybe provide an example where the above issue occurs?<|||||>@patil-suraj, you're 100% right!

I initially thought it's a problem because `EncoderDecoderConfig` does not have a `use_cache` param set to `True`, but it doesn't actually matter since `model.decoder.config.use_cache` will always be set to `True` by default which forces `use_cache` to be True in the decoder which makes it return the `past_key_values` => so all good then - thanks a lot for double-checking this :-)