Error while reproducing example for PerceiverForMultimodalAutoencoding
## Environment info
- `transformers` version: 4.14.1
- Platform: Linux-5.8.0-63-generic-x86_64-with-debian-bullseye-sid
- Python version: 3.7.6
- PyTorch version (GPU?): 1.9.0+cu111 (True)
- Tensorflow version (GPU?): 2.5.0 (False)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: no

### Who can help
@NielsRogge 

Trying to reproduce  [example](https://github.com/huggingface/transformers/blob/e51c7b5872785a74a03c011732173757d7c216c4/src/transformers/models/perceiver/modeling_perceiver.py#L1888) for `PerceiverForMultimodalAutoencoding` and getting:

```
>>> from transformers import PerceiverForMultimodalAutoencoding
2021-12-21 23:44:38.796858: I tensorflow/stream_executor/platform/default/dso_loader.cc:53] Successfully opened dynamic library libcudart.so.11.0
>>> import torch
>>> images = torch.randn((1, 16, 3, 224, 224))
>>> audio = torch.randn((1, 30720, 1))
>>> inputs = dict(image=images, audio=audio, label=torch.zeros((images.shape[0], 700)))
>>> model = PerceiverForMultimodalAutoencoding.from_pretrained('deepmind/multimodal-perceiver')
>>> outputs = model(inputs=inputs)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/tolik/anaconda3/lib/python3.7/site-packages/torch/nn/modules/module.py", line 1051, in _call_impl
    return forward_call(*input, **kwargs)
  File "/home/tolik/anaconda3/lib/python3.7/site-packages/transformers/models/perceiver/modeling_perceiver.py", line 1912, in forward
    return_dict=return_dict,
  File "/home/tolik/anaconda3/lib/python3.7/site-packages/torch/nn/modules/module.py", line 1051, in _call_impl
    return forward_call(*input, **kwargs)
  File "/home/tolik/anaconda3/lib/python3.7/site-packages/transformers/models/perceiver/modeling_perceiver.py", line 909, in forward
    inputs, modality_sizes, inputs_without_pos, subsampled_points=subsampled_output_points
  File "/home/tolik/anaconda3/lib/python3.7/site-packages/transformers/models/perceiver/modeling_perceiver.py", line 2429, in decoder_query
    [embed(modality, decoder_queries[modality]) for modality in sorted(self.modalities.keys())], dim=1
  File "/home/tolik/anaconda3/lib/python3.7/site-packages/transformers/models/perceiver/modeling_perceiver.py", line 2429, in <listcomp>
    [embed(modality, decoder_queries[modality]) for modality in sorted(self.modalities.keys())], dim=1
  File "/home/tolik/anaconda3/lib/python3.7/site-packages/transformers/models/perceiver/modeling_perceiver.py", line 2424, in embed
    pos = torch.broadcast_to(pos, [x.shape[0], x.shape[1], self.num_query_channels - x.shape[2]])
RuntimeError: The expanded size of the tensor (833) must match the existing size (831) at non-singleton dimension 2.  Target sizes: [1, 704, 833].  Tensor sizes: [1, 831]
```

