[Consistency] Automatically set decoder_input_ids for TFEncoderDecoderModel
# 🚀 Feature request

Currently, the `EncoderDecoderModel` class in PyTorch automatically creates the `decoder_input_ids` based on the `labels` provided by the user (similar to how this is done for T5/BART). This should also be implemented for `TFEncoderDecoderModel`, because currently users should manually provide `decoder_input_ids` to the model.

One can take a look at the TF implementation of BART for example to see how to shift the labels in order to automatically create the `decoder_input_ids`, namely [here](https://github.com/huggingface/transformers/blob/29dfb2dbb10cdba6327ff287db56b182c1db29b1/src/transformers/models/bart/modeling_tf_bart.py#L1120).

One should then also update the docstring correspondingly. 


Hi, I would like to work on this. Is there anyone already working on it?<|||||>No, you can work on it if you want!<|||||>Hi, Is it still open? I can work on this issue<|||||>There's a PR open (#14469), however it's not finished yet.<|||||>Hi @NielsRogge Can I finish up this issue with the closed PR as reference? I'd like to make a PR soon.<|||||>I believe this has already been resolved in #15175.