remove backend keys from FunctionalTensorWrapper, update TensorImpl::is_device methods (#81471)
It's kinda annoying to have wrapper subclass tensors (like `FunctionalTensorWrapper` include backend dispatch keys in their keyset, because when we occasionally write something buggy, we'll send the wrapper tensor the the backend kernel (which usually segfaults). By ensuring that wrapper tensors don't get backend keys, we'll get a nicer error when that happens.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/81471
Approved by: https://github.com/ezyang
