[bart] 2 SinusoidalPositionalEmbedding fixes
This PR:
* `embedding_dim` param for `SinusoidalPositionalEmbedding` can now be odd.
* fixes a bug "RuntimeError: a view of a leaf Variable that requires grad is being used in an in-place operation" appearing in pytorch-1.8+ (this var requires no grad, so make it so before we do anything grad-related with it).

Fixes: #8021

@sshleifer, @LysandreJik 