Don't use PrimTorch decomposition for empty (#94512)
This PR removes the unnecessary == 0 guard when constructing empty tensors, by ensuring that when we create a contiguous tensor we go directly to the C++ torch.empty implementation (instead of indirecting through empty_strided), where we can bypass doing zero tests when computing the size of the storage. This probably also speeds up trace time.
When I did this, I found out that `empty_tensor_restride_symint` was flagrantly wrong (we had never exercised it before because we redirected to `empty_strided` in PrimTorch decomp, which doesn't hit this codepath.) The bugs:
* Stride computation was wrong (only `last_idx` was ever written to)
* Using set_sizes_and_strides with `sym_sizes` input doesn't work, because there is some sort of ordering problem where `clone_symvec` isn't safe when you clone a vector into itself. Probably should fix this.
Signed-off-by: Edward Z. Yang <ezyang@meta.com>
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94512
Approved by: https://github.com/ngimel
