[feature] Shots backend (#130)

# Description

- Refactors `CuTensorNetBackend` into `CuTensorNetStateBackend` which
now uses the new `GeneralState`.
- Introduces a new `CuTensorNetShotsBackend` for shot-based simulation.

# Related issues

Completes #121.

# Checklist

- [x] I have run the tests on a device with GPUs.
- [x] I have performed a self-review of my code.
- [x] I have commented hard-to-understand parts of my code.
- [x] I have made corresponding changes to the public API documentation.
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have updated the changelog with any user-facing changes.

docs/changelog.rst

@@ -4,7 +4,14 @@ Changelog
 Unreleased
 ----------
 
+* API breaking changes
+    * Renamed ``CuTensorNetBackend`` to ``CuTensorNetStateBackend``.
+    * Moved ``get_operator_expectation_value`` and ``get_circuit_overlap`` from ``backends`` submodule to ``general_state`` submodule.
+    * **Warning** ``TensorNetwork`` object will soon be deprecated in favour of the new ``GeneralState``.
+
+
 * New API: ``GeneralState`` for exact simulation of circuits via contraction-path optimisation. Currently supports ``get_statevector()``, ``expectation_value()`` and ``sample()``.
+* New feature: ``CuTensorNetShotsBackend`` for simulation of circuit shots.
 * New feature: ``add_qubit`` to add fresh qubits at specified positions in an ``MPS``.
 * New feature: added an option to ``measure`` to toggle destructive measurement on/off. Currently only supported for ``MPS``.
 * New feature: a seed can now be provided to ``Config`` objects, providing reproducibility across ``StructuredState`` simulations.

docs/modules/general_state.rst

@@ -20,13 +20,17 @@ cuQuantum `contract` API interface
 
 .. autoclass:: pytket.extensions.cutensornet.general_state.ExpectationValueTensorNetwork
 
+.. autofunction:: pytket.extensions.cutensornet.general_state.tk_to_tensor_network
+
 .. autofunction:: pytket.extensions.cutensornet.general_state.measure_qubits_state
 
-.. autofunction:: pytket.extensions.cutensornet.general_state.tk_to_tensor_network
+.. autofunction:: pytket.extensions.cutensornet.general_state.get_operator_expectation_value
+
+.. autofunction:: pytket.extensions.cutensornet.general_state.get_circuit_overlap
 
 
 Pytket backend
 ~~~~~~~~~~~~~~
 
 .. automodule:: pytket.extensions.cutensornet
-    :members: CuTensorNetBackend
+    :members: CuTensorNetStateBackend, CuTensorNetShotsBackend

pytket/extensions/cutensornet/__init__.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 """Module for conversion from tket primitives to cuQuantum primitives."""
 
-from .backends import CuTensorNetBackend
+from .backends import CuTensorNetStateBackend, CuTensorNetShotsBackend
 from .general import CuTensorNetHandle
 
 # _metadata.py is copied to the folder after installation.

pytket/extensions/cutensornet/backends/__init__.py

@@ -13,4 +13,4 @@
 # limitations under the License.
 """Backend for utilising the cuQuantum simulator directly from pytket"""
 
-from .cutensornet_backend import CuTensorNetBackend
+from .cutensornet_backend import CuTensorNetStateBackend, CuTensorNetShotsBackend