FSTM compatibility issues with other `ForConditionalGeneration` models
### Description

The implementation of FSMT models like [`facebook/wmt19-en-de`](https://huggingface.co/facebook/wmt19-en-de) is atypical with respect to different aspects that are normally supported in other `ForConditionalGeneration` models. In particular:

- Both `FSMTModel` and `FSMTForConditionalGeneration` lack utility methods like `get_encoder` and `get_decoder`
- `FSMTForConditionalGeneration` and all its subclasses do not accept `inputs_embeds` and `decoder_inputs_embeds` as possible alternatives to `input_ids` and `decoder_input_ids` for the forward pass.

In particular, this renders the model unusable when using feature attribution methods through the `inseq` library (see related issue inseq-team/inseq#153).

### Expected behavior

We would expect all models belonging to the same family to expose a consistent API for external usage.

### System Info

- `transformers` version: 4.24.0
- Platform: Linux-5.15.0-56-generic-x86_64-with-glibc2.29
- Python version: 3.8.10
- Huggingface_hub version: 0.11.1
- PyTorch version (GPU?): 1.13.0+cu117 (True)
- Tensorflow version (GPU?): not installed (NA)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: NA
- Using distributed or parallel set-up in script?: NA

### Who can help?

@ArthurZucker @younesbelkada 