Make constant folder create initializers

[justinchuby]

Right now all folded constants are created as Constant nodes, and initializers are always turned into a constant node even if they cannot be folded.

Make constant folder create initializers and allow a lazy evaluate mode to avoid copying data.

[titaiwangms]

@justinchuby

I think the description might be a bit inaccurate, as according to the code here, it looks like only the initializers with op.Transpose (_DEFAULT_ALWAYS_FOLD_OPS) will be folded (assuming the task here is to speedup large weights.)? Is that the only targeted initializers here?

[titaiwangms]

Main goal: Do not create new constant code, create initializer instead.