Fixes https://github.com/pytorch/pytorch/issues/159995
Currently there are two problems with extern kernels in subgraphs:
1. They don't get serialized to the extern kernel json file because we only look at the toplevel graph.
2. Since the scope of each extern_kernel list is within its own subgraph, the indices referencing the operator is messed up because each subgraph will start counting from 0.
So, this PR moves the extern_kernels list to a global view (under virtualized) so that we can count the extern kernels across subgraphs and the toplevel graph.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/160004
Approved by: https://github.com/ydwu4
3c8c509a9c