sym_numel (#82374)
### Description
This PR makes `numel` symint-aware similar to `sym_sizes()` and `sym_strides()`. Similar to https://github.com/pytorch/pytorch/pull/81300 . This PR is the part of a bigger project to support dynamic_shapes.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/82374
Approved by: https://github.com/ezyang
