Fix weight loading issue
# What does this PR do?

Fix #14002 + add 2 more tests

(I uploaded the converted TF checkpoint to [ydshieh](https://huggingface.co/ydshieh/bert2bert-cnn_dailymail-fp16)). Might be better for @patrickvonplaten to upload to [patrickvonplaten](https://huggingface.co/patrickvonplaten/bert2bert-cnn_dailymail-fp16/blob/main/README.md)

## Who can review?

@Rocketknight1 @patrickvonplaten @LysandreJik 
The issue in #14002 comes from the fact, when using `from_pt=True`, the block shown at the end doesn't use `load_weight_prefix`.

However, it is required to extend the variable scope for the 2 components (encoder & decoder) of a TF composite models - to make the subsequent `save_pretrained -> from_pretrained` work after a creation from `from_encoder_decoder_pretrained`.

I feel it would be better to modify `load_pytorch_weights_in_tf2_model` to address this situation, but I tried to avoid modify this Hugging Face's TF core method.

https://github.com/huggingface/transformers/blob/d5b82bb70c2e8c4b184a6f2a7d1c91d7fd156956/src/transformers/modeling_tf_utils.py#L1445-L1449
<|||||>Looks great to me!<|||||>Hi, I just realized that `TFEncoderDecoderModel` is released with `v4.12.0` without this fix being merged.