Issue in wav2vec2ForPretraining
### System Info

```shell
In the example mentioned in the doc, when I print the loss, it is "None"
```


### Who can help?

@patrickvonplaten

### Information

- [X] The official example scripts
- [ ] My own modified scripts

### Tasks

- [X] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

import torch
from transformers import AutoFeatureExtractor, Wav2Vec2ForPreTraining
from transformers.models.wav2vec2.modeling_wav2vec2 import _compute_mask_indices
from datasets import load_dataset

feature_extractor = AutoFeatureExtractor.from_pretrained("facebook/wav2vec2-base")
model = Wav2Vec2ForPreTraining.from_pretrained("facebook/wav2vec2-base")

ds = load_dataset("hf-internal-testing/librispeech_asr_dummy", "clean", split="validation")
input_values = feature_extractor(ds[0]["audio"]["array"], return_tensors="pt").input_values  # Batch size 1

# compute masked indices
batch_size, raw_sequence_length = input_values.shape
sequence_length = model._get_feat_extract_output_lengths(raw_sequence_length)
mask_time_indices = _compute_mask_indices((batch_size, sequence_length), mask_prob=0.2, mask_length=2)
mask_time_indices = torch.tensor(mask_time_indices, device=input_values.device, dtype=torch.long)

with torch.no_grad():
    outputs = model(input_values, mask_time_indices=mask_time_indices)

# compute cosine similarity between predicted (=projected_states) and target (=projected_quantized_states)
cosine_sim = torch.cosine_similarity(outputs.projected_states, outputs.projected_quantized_states, dim=-1)

# show that cosine similarity is much higher than random
cosine_sim[mask_time_indices.to(torch.bool)].mean() > 0.5

# for contrastive loss training model should be put into train mode
model = model.train()
loss = model(input_values, mask_time_indices=mask_time_indices).loss

### Expected behavior

```shell
Modify the example doc
```

Hey @annihi1ation,

What `transfromres` version are you using?<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.