make states contiguous for past_key_values
Add contiguous for key and value states.

If someone uses past_key_values, it seems to raise the following exception:

`
RuntimeError: view size is not compatible with input tensor‘s size and stride ...
`
since bart executes torch.cat in BartAttention class:

`
key_states = torch.cat([past_key_value[0], key_states], dim=2)
`
`
value_states = torch.cat([past_key_value[1], value_states], dim=2)
`
Thus, we should make key_states and value_states contiguous.


@patrickvonplaten

---
More, bart cannot correctly process the length of attention_mask when the item of past_key_values is added.


_The documentation is not available anymore as the PR was closed or merged._