Fix M2M100 positional embedding creation for ONNX
# What does this PR do?

This PR changes the reshape step when computing the sinusoidal positional embeddings in M2M100 to make it work with ONNX.

Shape inference is incorrect before:

![Capture d’écran du 2023-01-26 18-12-40](https://user-images.githubusercontent.com/25418079/214904210-64dc5fd9-a84b-4cee-98f2-87626b721a95.png)

You can see that ONNX sets the last axis of the shape of `last_hidden_state` to be dynamic, with some auto-generated name, while it should be static.

After the fix:
![Capture d’écran du 2023-01-26 18-12-40](https://user-images.githubusercontent.com/25418079/214904432-9682bec6-aef6-47e6-a9b6-505730736dee.png)
_The documentation is not available anymore as the PR was closed or merged._