Fix doc example: mask_time_indices (numpy) has no attribute 'to'
# What does this PR do?

In speech to text models, there are doc examples

```
    >>> mask_time_indices = _compute_mask_indices((batch_size, sequence_length), mask_prob=0.2, mask_length=2)
    >>> with torch.no_grad():
    ...     outputs = model(input_values, mask_time_indices=mask_time_indices)
```
which gives `numpy.ndarray`, and the following line fails

https://github.com/huggingface/transformers/blob/19d37c2dd36d73537a6855c56808e524fb584459/src/transformers/models/wav2vec2/modeling_wav2vec2.py#L1481

with `AttributeError: 'numpy.ndarray' object has no attribute 'to'`.

This PR add 

```
mask_time_indices = torch.tensor(mask_time_indices, device=input_values.device, dtype=torch.bool)
```
in the examples to make them work.


## Who can review?

@patrickvonplaten 
Thank you!