Handling initializers in GraphBuilder (#2889)

(a) Update the logic for creating initializers to ensure that they are
always added to the root (main) Graph.
(b) Add a utility to convert initializers into Constant nodes (which is
necessary for a Function in ONNX).

Note: An alternative considered was adding an option to the GraphBuilder
so that we automatically construct Constant nodes instead of
initializers when the options says so. While that avoids the extra pass
in the end, it has some minor implications to what the graph would look
like (in terms of whether we want all Constant nodes upfront in one
place, and what kind of node-names (based on node numbering) we generate
etc. Hence, leaving it in the current form. But it can be changed as
above if desirable.

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Signed-off-by: G Ramalingam <grama@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: gramalingam