Fix `WhisperModelTest` 
# What does this PR do?


### Fix `test_model_parallelism` for `Whisper`
A merged PR `Add WhisperTokenizerFast` (#21222)  (02/21) started failing `test_model_parallelism` .
That PR didn't change relevant test/modeling file, but just changed the model tester's vocab size from `99` to `200`.

When I traced it, I found at this places inside `WhisperDecoder`
```python
inputs_embeds = self.embed_tokens(input_ids)
positions = self.embed_positions(input_ids, ...)
hidden_states = inputs_embeds + positions
```
- in previous commit,  `embed_tokens` and `embed_positions` weight matrices are on the same GPU `1`.
- After that PR, one is on GPU `0` another being on GPU `1`. 

I fixed the issue by adding

```python
    # Needs higher percentages after model tester's vocab_size is changed to 200 (PR #21222)
    model_split_percents = [0.8, 0.9]
```

### Fix `test_torchscript_*` for `Whisper`

PR #21298 added an optional argument `attention_mask` to `WhisperModel` model classes. `torchscript` tests need a bit change to make it work.
_The documentation is not available anymore as the PR was closed or merged._