PyTorch -> MDF pipeline

[pgleeson]

@davidt0x @patrickstock Can you add details on status?

[davidt0x]

Rough version of the torchscript to MDF translator in modeci_mdf.export.torchscript. Still a work in progress.

• Parsing and translation of TorchScript IR graph to MDF
• Removal of constant TorchScript nodes (they become parameters in MDF)
• No execution, the MDF is full of TorchScript operators, working on this now.
• Consolidation (maybe removal) of some primitive operators that TorchScript uses a lot (prim::ListConstruct, prim::TupleConstruct). I think we can flesh out the MDF spec a bit more to allow tuples and lists of parameters and input ports to be passed as arguments to functions. I think this would allow us to remove a lot these.
• Control flow is still not handled really. Loop and If operators are translated but the blocks (subgraphs that contain their computations) are not being recursively generated in MDF. I had something working but there is a bug. Need to think about this a bit in light of the conditions stuff.