unclear `prepare_seq2seq_batch` deprecation
When using `prepare_seq2seq_batch` the user now gets:

> transformers-master/src/transformers/tokenization_utils_base.py:3277: FutureWarning: `prepare_seq2seq_batch` is deprecated and will be removed in version 5 of 🤗 Transformers. Use the regular `__call__` method to prepare your inputs and the tokenizer under the `with_target_tokenizer` context manager to prepare your targets. See the documentation of your specific tokenizer for more details.

It's very hard to act on as, I'm not sure what "regular `__call__` method" refers to and I could find any tokenizer documentation that ever mentions `with_target_tokenizer`.

Perhaps this is an unintended typo? was it meant to be `with target_tokenizer`? `with FooTokenizer`?

Please kindly suggest a more user-friendly deprecation and at least one example or a link to such.

Thank you.

@sgugger, @LysandreJik 
Why is `__call__` hard to understand? It's the regular Python method for when the tokenizer is called directly on inputs. How would you formulate that better?

For the `with_target_tokenizer` it's a typo indeed, it should be `as_target_tokenizer`.

As for an example, this is what is used in every example script, see for instance the [run_translation](https://github.com/huggingface/transformers/blob/9adff7a0f49f88a6cc718a1d30088988dc78bb6a/examples/pytorch/translation/run_translation.py#L414) script. 

I'm curious, where did you still find a reference to this method? It's been removed from all examples and documentation normally (and has been deprecated five months ago).<|||||>It's totally obvious once I see an example that I was now able to find since you gave the correct context manager name, it is so not obvious from the warning message. Moreover, none of the tokenizers document that as suggested by the warning's message. They do document the specifics of its usage.

I made an attempt at another version here: https://github.com/huggingface/transformers/pull/12669<|||||>> I'm curious, where did you still find a reference to this method? It's been removed from all examples and documentation normally (and has been deprecated five months ago).

In several of the scripts I used in the past to make tiny models.

I'm curious in turn why was this wrapper deprecated? To make things more explicit? Looks like a lot more code to write instead of the wrapper.
<|||||>I stumbled upon this issue when googling the warning. For the translation task this
`tokenized_text = tokenizer.prepare_seq2seq_batch([text], return_tensors='pt')`
has to be replaced by this:
```
with tokenizer.as_target_tokenizer():
    tokenized_text = tokenizer(text, return_tensors='pt')
```
Which is much clearer than using `prepare_seq2seq_batch`, but for anyone coming from other languages but python, the concept of `__call__`  might not be transparent in first place :)<|||||>I'm getting the same text, not the translated one when I change from `prepare_seq2seq_batch` to `as_target_tokenizer`