Add deprecation warning to algs module (#10406)

* Add deprecation warning to algs module

* Apply suggestions from code review

Co-authored-by: Eric Arellano <14852634+Eric-Arellano@users.noreply.github.com>

* Add link to repo

* Add release note

* Add migration guide to release note

* Apply suggestions from code review

Co-authored-by: Matthew Treinish <mtreinish@kortar.org>

---------

Co-authored-by: Eric Arellano <14852634+Eric-Arellano@users.noreply.github.com>
Co-authored-by: Matthew Treinish <mtreinish@kortar.org>
Co-authored-by: Steve Wood <40241007+woodsp-ibm@users.noreply.github.com>

qiskit/algorithms/__init__.py

@@ -14,6 +14,15 @@
 =====================================
 Algorithms (:mod:`qiskit.algorithms`)
 =====================================
+
+.. deprecated:: 0.25.0
+
+    The :mod:`qiskit.algorithms` module has been migrated to an independent package:
+    https://github.com/qiskit-community/qiskit-algorithms.
+    The current import path is deprecated and will be removed no earlier
+    than 3 months after the release date. You can run ``pip install qiskit_algorithms``
+    and import ``from qiskit_algorithms`` instead.
+
 It contains a collection of quantum algorithms, for use with quantum computers, to
 carry out research and investigate how to solve problems in different domains on
 near-term quantum devices with short depth circuits.
@@ -297,6 +306,8 @@
    AlgorithmJob
 
 """
+import warnings
+
 from .algorithm_job import AlgorithmJob
 from .algorithm_result import AlgorithmResult
 from .evolvers import EvolutionResult, EvolutionProblem
@@ -412,3 +423,13 @@
     "VarQTE",
     "VarQTEResult",
 ]
+
+warnings.warn(
+    "``qiskit.algorithms`` has been migrated to an independent package: "
+    "https://github.com/qiskit-community/qiskit-algorithms. "
+    "The ``qiskit.algorithms`` import path is deprecated as of qiskit-terra 0.25.0 and "
+    "will be removed no earlier than 3 months after the release date. "
+    "Please run ``pip install qiskit_algorithms`` and use ``import qiskit_algorithms`` instead.",
+    category=DeprecationWarning,
+    stacklevel=2,
+)

releasenotes/notes/deprecate-algorithms-7149dee2da586549.yaml

@@ -0,0 +1,22 @@
+---
+deprecations:
+  - |
+    The :mod:`qiskit.algorithms` module has been deprecated and will be removed
+    in a future release. It has been superseded by a new standalone library
+    ``qiskit-algorithms`` which can be found on PyPi or on Github here:
+
+    https://github.com/qiskit-community/qiskit-algorithms
+
+    The :mod:`qiskit.algorithms` will continue to work as before and bug fixes
+    will be made to module until its future removal, but active development
+    of new features has moved to the new package.
+    If you're relying on :mod:`qiskit.algorithms` you should update your
+    requirements to also include ``qiskit-algorithms`` and update the imports
+    from ``qiskit.algorithms`` to ``qiskit_algorithms``. If you have not yet
+    migrated from ``QuantumInstance``-based to primitives-based algorithms,
+    you should follow the migration guidelines in https://qisk.it/algo_migration.
+    The decision to migrate the :mod:`~.algorithms` module to a 
+    separate package was made to clarify the purpose Qiskit and 
+    make a distinction between the tools and libraries built on top of it.
+
+