[dynamo] Add dynamic shapes support to torch.Size.numel (#108239)
Currently numel only supports static shapes, but this expands it to support
generating symbolic arithmetic into the graph. e.g.
```
# x.size().numel with x.size() = [s0, 1, s1]
size = l_x_.size()
getitem = size[0]
getitem_2 = size[2]; size = None
mul = getitem * getitem_2; getitem = getitem_2 = None
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/108239
Approved by: https://github.com/ezyang
