Is your feature request related to a problem? Please describe.
Currently, graphix does not assume arbitrary initial state for the MBQC. This is not a problem (still universal, just like real quantum hardware) but it would be a good feature to have, to test out MBQC with small toy models

Describe the feature you'd like
An option to simulate with arbitrary input state. For this, we would need:

• an option to keep the input nodes during the Pattern.perform_pauli_measurements(). Otherwise input nodes will be considered fixed |+> states and be preprocessed, making it difficult to simulate with arbitrary input state.
• an option to use arbitrary input statevector in StatevectorBackend.
• similar option for TN backend.

Additional context
na