skottmckay/bugfix/SubgraphInput (#4004)
Description:
Fix 2 edge cases as described here: #3755 (comment)
Create a NodeArg for subgraph inputs even if they have no type. If they are only used as an implicit input to another level of nested subgraph we will not create a NodeArg via any other path
Allow an If output to have no shape. Obscure edge case where a loop carried dependency to a Loop node is passed through a nested If node subgraph (i.e. the Loop subgraph contains an If node with a nested subgraph for the else_branch/then_branch). We can't infer a shape for a loop carried dependency (they may change across iterations), which means we can't infer a shape for the nested If subgraph output either. We have delayed allocation support for If outputs so use that.
Motivation and Context
#3755
