fixed whisper positional encoding
Whisper Positional Encoding has incorrect behavior when passing inputs_embeds:
- When we are passing `input_ids` (batch_size x seq_len) it takes the -1 dimension of it which is correct.
- When we are passing `input_embeds` (batch_size x seq_len x embedding_dim) it doesn't work. If we take -1 dimension, we get embedding dim elements.

My fix is just to take the 1 dimension which will be always correct




_The documentation is not available anymore as the PR was closed or merged._<|||||>@clefourrier @gante<|||||>Looks like, there are no problems with tests.
![image](https://user-images.githubusercontent.com/43551010/236498753-3a084a00-873a-4c91-b9af-caf5a12481e2.png)
