Fix Bug in Flax Seq2Seq Models
This PR makes two changes to each of `FlaxEncoderDecoderModel` and `FlaxSpeechEncoderDecoderModel`:

1. Amends the input docstrings to remove incorrect information about the model "_shifting tokens right for denoising_". In Flax, `decoder_input_ids` are obtained by shifting the target labels right **outside** of the seq2seq model, not **within** as stated in the docstrings.
2. Raises a `ValueError` if `decoder_input_ids` are not provided. The current behaviour allows for `decoder_input_ids` to be omitted, in which case they default to `None`. This causes errors when `decoder_input_ids=None` is manipulated with JAX functions to build the `decoder_attention_mask` and `decoder_position_ids` should they be omitted from the arguments too.

The following code snippet throws the error aforementioned in 2:
```python
from transformers import FlaxSpeechEncoderDecoderModel
import jax.numpy as jnp
model = FlaxSpeechEncoderDecoderModel.from_encoder_decoder_pretrained('hf-internal-testing/tiny-random-wav2vec2', 'hf-internal-testing/tiny-random-gpt2', encoder_from_pt=True, decoder_from_pt=True)
inputs = jnp.ones((2, 5000), dtype=jnp.float32)
outputs = model(inputs)
```

Output:
```python
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/sanchitgandhi/transformers/src/transformers/models/speech_encoder_decoder/modeling_flax_speech_encoder_decoder.py", line 688, in __call__
    decoder_attention_mask = jnp.ones_like(decoder_input_ids)
  File "/Users/sanchitgandhi/venv/lib/python3.8/site-packages/jax/_src/numpy/lax_numpy.py", line 3706, in ones_like
    _check_arraylike("ones_like", a)
  File "/Users/sanchitgandhi/venv/lib/python3.8/site-packages/jax/_src/numpy/lax_numpy.py", line 570, in _check_arraylike
    raise TypeError(msg.format(fun_name, type(arg), pos))
TypeError: ones_like requires ndarray or scalar arguments, got <class 'NoneType'> at position 0.
```
_The documentation is not available anymore as the PR was closed or merged._