Fix from_pretrained when model is instantiated on the meta device
# What does this PR do?

#22437 broke the `from_pretrained` method whenever the model is instantiated on the meta device and the state dict passed is not complete (see [this issue](https://github.com/huggingface/accelerate/issues/1333) for one example).

Basically the check will remove all keys from `missing_keys` since all parameters on the meta device share the same data pointer. I had advocated to use another solution in that PR but the contributor did not listen

Since we rely on those `missing_keys` later on to re-initialize the weights that are not in the state dict, the model ends up with weights on the meta device.
_The documentation is not available anymore as the PR was closed or merged._