[M2M100] fix positional embeddings
# What does this PR do?

The torchscript tests for `M2M100` are failing on master. This is because the `weights` in `M2M100SinusoidalPositionalEmbedding` are initially not on the same device as the rest of the parameters. 

The PR makes the `weights` as `nn.Parameter` so they'll be on the same device.