Fix doc examples: name '...' is not defined
# What does this PR do?

For some doc examples in PT model files, we get exception: `NameError: name '...' is not defined`

This PR fixes this issue.

In `modeling_visual_bert.py`, there are some places using an assumed-existing function `get_visual_embeddings`.

```
# Assumption: `get_visual_embeddings(image)` gets the visual embeddings of the image in the batch.
visual_embeds = get_visual_embeddings(image).unsqueeze(0)
```
These places won't pass the doc example test - so need to a way to not run against them.
(Or an example implementation for this function should be provided)

## Who can review?

@sgugger 
> Thanks for fixing. For the one in visual BERT, it shouldn't be run by doctest, so the `>>>` should be removed.

I removed `>>>` and `...` as suggested.<|||||>Thnaks!