Add create_mutable_buffer util

Rationale:
    * We want to support stateful custom ops inserted after to_edge in a
      non delegated cases like op-lib-backends i.e. cortex-m::cmsis_nn::linear

Details:
    * Allows a pass to add a mutable buffer.
    * Essentially redoes what export, to_edge does for a buffer
      registered in an nn.module.
    * Also plays nice with to_executorch passes like conversion to
      in-place, and write-back for mutated buffers.
    * Verifies above with tests, adds a HelperPass for someone looking
      to leverage this util as an example too.

To Test:
    $ python -m unittest backends.transforms.test.test_create_mutable_buffer

Author: digantdesai