Replace false parameter by a buffer
# What does this PR do?

The weights of the sinusoidal embedding are defined as a parameter with no grad in M2M100 (and thus XGLM), and never saved in the state dict. The problem is that when loading this with `low_cpu_mem_usage=True`, this false parameter will be replaced by an empty weight on the meta device, which is not re-initialized afterward (since it's not in the state dict). As a result, the model is not usable if `low_cpu_mem_usage=True` is used.

By replacing it with a buffer, the weight is ignore by init_empty_weights and thus preserved.
_The documentation is not available anymore as the PR was closed or merged._