Extend constant prop pass to work with int/float/etc scalars and fix input specs. (#2950)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2950

1. Cleanup / Refactor constant prop pass.

2. Enable constant propagation for ops with constant scalar arguments -- int/float/dtype/bool/str.
Nodes of type `Op(constant_tensor, some_int, some_float, some_dtype, ...)` can now be constant propagated.

3. Fix order of input spec to match the expected spec in `ExportGraphSignature` class.
parameters->buffers->constants->user_inputs.
Before this diff, input_specs for the newly added constant tensors were appended to graph_signature, which would cause failures.

Reviewed By: dulinriley

Differential Revision: D55891278

fbshipit-source-id: fe1867cb6a99d0140d6a2e076027688cb1ddc0cd