Fix for https://github.com/llvm/torch-mlir/issues/2765
The onnx docs say that you can't do shape inference using the in-memory
API for models > 2 GB. This fix replaces that API with the file-based
API. Since the new API generates an intermediate file, also added a
--keep switch to keep that file, which I delete by default.
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Co-authored-by: Dave Liddell <dliddell@xilinx.com>