GPT-J specific half precision on CPU note
This PR adds a note to the GPT-J model doc indicating that the half precision on CPU example is specific to the model, and doesn't generally apply. Related to https://github.com/huggingface/transformers/issues/21989
_The documentation is not available anymore as the PR was closed or merged._<|||||>My bad, I misunderstood the interaction in the issue. Tried the example, and it does indeed not work. I'll do another pass.<|||||>Did another take. It should now be clear that half-precision works on CUDA devices only. 

Btw, not sure if this is relevant but I tried this example without explicitly sending to device on a GPU, and it threw the same error. 
