PyTorch can't depend on `fbgemm_gpu` as a dependency because `fbgemm_gpu` already has a dependency on PyTorch. So this PR copy / pastes kernels from `fbgemm_gpu`:
* `dense_to_jagged_forward()` as CUDA registration for new ATen op `_padded_dense_to_jagged_forward()`
* `jagged_to_padded_dense_forward()` as CUDA registration for new ATen op `_jagged_to_padded_dense_forward()`
CPU impls for these new ATen ops will be added in a follow-up PR.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/125946
Approved by: https://github.com/davidberard98