Change a parameter name in FlaxBartForConditionalGeneration.decode()
# What does this PR do?

In short: Change a parameter name in `FlaxBartForConditionalGeneration.decode()`: `deterministic` -> `train`.

In the current version of `FlaxBartForConditionalGeneration.decode()` method, it takes an argument `deterministic`, while 
`FlaxBartPreTrainedModel.decode()`, `FlaxT5PreTrainedModel.decode()`, `FlaxT5ForConditionalGeneration.decode()`, and similar places in `FlaxGPT2`, they all use `train` as the argument.

It seems to me that there is a (implicit?) convention that, in Flax models, we use `deterministic` parameter for `nn.Module` and parameter `train` for models inheriting from `FlaxPreTrainedModel`.

This PR fix this small inconsistency in `FlaxBartForConditionalGeneration.decode()`. I hope this PR makes sense, despite the change is really small.

## Before submitting

- [ x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests)

## Who can review?

@patrickvonplaten 
@patil-suraj 



I made the same change to flax marian and mbart as suggested.<|||||>Thanks a lot for fixing this!

Merging, the failing test is un-related. 