[export] Preserve meta["val"] on placeholders in dynamo.export(). (#88651)
Summary:
Today when we transform the captured graph in the last step in export(aten_graph=True), we construct a new graph which doesn't have the all the metadata to be preserved, for example, node.meta["val"].
meta["val"] is important for writing passes and analysis on the graph later in the pipeline, we may want to preserve that on placeholder nodes.
Test Plan: test_export.py:test_export_meta_val
Differential Revision: D41110864
Pull Request resolved: https://github.com/pytorch/pytorch/pull/88651
Approved by: https://github.com/tugsbayasgalan, https://github.com/jansel
