Return symbolic expression as a raw Sympy or Symengine object.
Symengine is used preferentially; if both are available, the result will always be a
symengineobject. Symengine is a separate library but has integration with Sympy.
This is for interoperability only. Qiskit will not accept or work with raw Sympy or Symegine expressions in its parameters, because they do not contain the tracking information used in circuit-parameter binding and assignment.