fix forward _partition_and_lower_one_graph_module

Summary:
D60296909 broke some executorch backend code.

Note that `inputs_to_buffers`, `buffers`, `inputs_to_parameters`, `parameters` on a graph signature were all property accessors even before that diff, so mutating operations like `pop()` and `remove()` had no effect since a new list / dict was returned on every property access.
That diff makes them immutable so `pop()` and `remove()` are now errors.

Reviewed By: pssrawat

Differential Revision: D60642036

fbshipit-source-id: 7e65f333acda59d99d541ef3f3c4be008d10572f