[Wav2Vec2] Fix None loss in doc examples
# What does this PR do?

Doc examples in [Wav2Vec2ForPreTraining](https://huggingface.co/docs/transformers/model_doc/wav2vec2#transformers.Wav2Vec2ForPreTraining.forward.example) and  [Wav2Vec2ConformerForPreTraining](https://huggingface.co/docs/transformers/model_doc/wav2vec2-conformer#transformers.Wav2Vec2ConformerForPreTraining.forward.example) produce None loss due to missing sampled_negative_indices parameter in the model. See #15232 

* pass sampled_negative_indices parameter to the model to avoid getting a None loss
* The sequence length is a tensor when it should be an integer. Add .item() call to address this issue.

Fixes #15232 


## Who can review?

@patrickvonplaten 




_The documentation is not available anymore as the PR was closed or merged._<|||||>@patrickvonplaten Could you take a look at this PR for Wav2Vec2, thanks :-)