TensorImpl:::size_custom to support NestedTensor.size (#80236)
This allows subclasses such as NestedTensorImpl to provide special behavior for `int64_t size(int64_t d)` that'll also be accessible by our Python frontend.
It follows the same pattern as sizes_custom.
Currently getting CI before asking for a review.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80236
Approved by: https://github.com/ezyang
