docs(reuse): add User Guide page on reusing functions and nodes (#1045)

Walks through the four Hamilton patterns for reusing the same logic
across different data sources, regions, or runtime contexts:

- driving the same function module from multiple Drivers
  (examples/feature_engineering/feature_engineering_multiple_contexts/)
- Builder.allow_module_overrides() for swapping same-named functions
  (examples/module_overrides/)
- @subdag for repeating a transformation graph inside one Driver
  (examples/reusing_functions/)
- @parameterized_subdag for many subdags at once, with the readability
  warning quoted verbatim from the source docstring

Every snippet on the page is a literalinclude pulled from the example
files, so the docs and the runnable code can't drift apart.

Closes #1045

Signed-off-by: Olajumoke Akinremi <106763970+Hore01@users.noreply.github.com>

Author: Hore01

docs/how-tos/index.rst

@@ -12,6 +12,7 @@ directory. If there's an example you want but don't see, reach out or open an is
    load-data
    caching-tutorial
    use-for-feature-engineering
+   reuse-nodes
    ml-training
    llm-workflows
    run-data-quality-checks

docs/how-tos/reuse-nodes.rst

@@ -0,0 +1,262 @@
+..
+    Licensed to the Apache Software Foundation (ASF) under one
+    or more contributor license agreements.  See the NOTICE file
+    distributed with this work for additional information
+    regarding copyright ownership.  The ASF licenses this file
+    to you under the Apache License, Version 2.0 (the
+    "License"); you may not use this file except in compliance
+    with the License.  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+    Unless required by applicable law or agreed to in writing,
+    software distributed under the License is distributed on an
+    "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+    KIND, either express or implied.  See the License for the
+    specific language governing permissions and limitations
+    under the License.
+
+==========================================
+Reusing functions and nodes
+==========================================
+
+A common question on `Slack <https://join.slack.com/t/hamilton-opensource/shared_invite/zt-2niepkra8-DGKGf_tTYhXuJWBTXtIs4g>`_:
+*"I want to run the same logic for several regions / datasets / model variants
+-- what is the Hamilton way?"* Hamilton has four answers, and the right one
+depends on how the variation is shaped.
+
+This page walks through them in order from simplest to most advanced:
+
+1. **Reuse a function module across multiple Drivers** -- the data is what
+   varies, the dataflow is the same.
+2. **Override a module with another that has the same function names** --
+   one or two specific functions need to be swapped (e.g. for testing or for a
+   different runtime context).
+3. **Use** ``@subdag`` -- you want the *same* transformation graph evaluated
+   several times *inside one Driver*, with different inputs or config.
+4. **Use** ``@parameterized_subdag`` -- the variation is large enough that
+   writing one ``@subdag`` per case becomes tedious. (Advanced.)
+
+Every code sample below is taken from a runnable example in the
+`examples folder <https://github.com/apache/hamilton/tree/main/examples>`_, so
+you can copy any of them, run them locally, and adapt them.
+
+
+1. Reuse a function module across multiple Drivers
+--------------------------------------------------
+
+If the *dataflow* is the same and only the *data* changes, you do not need
+any decorator -- you just import the function module and build a Driver
+wherever you need one. This is the most common form of reuse and the one to
+reach for first.
+
+The
+`feature_engineering_multiple_contexts <https://github.com/apache/hamilton/tree/main/examples/feature_engineering/feature_engineering_multiple_contexts>`_
+example shows this pattern across an offline ETL and an online FastAPI
+service: ``features.py`` is written **once**, then driven from two contexts.
+The offline ETL builds a Driver and executes it on a batch of rows; the
+online server builds another Driver from the same module and executes it
+per-request.
+
+When to reach for this pattern:
+
+* The same feature definitions need to run in batch *and* in a request
+  handler.
+* You want to share code between training and inference.
+* You want different teams to consume the same canonical module with their
+  own inputs.
+
+What you do *not* need: any Hamilton-specific decorator. The reuse is just
+ordinary Python imports plus building a Driver per context.
+
+
+2. Override a module to swap same-named functions
+-------------------------------------------------
+
+Sometimes you want most of a dataflow to stay the same and only swap one
+or two functions -- for example, replacing a real data loader with a mock
+one in tests, or switching between two implementations of the same business
+rule.
+
+By default Hamilton refuses to build a DAG when two modules define
+functions with the same name, because the resulting graph would be
+ambiguous. The
+`module_overrides <https://github.com/apache/hamilton/tree/main/examples/module_overrides>`_
+example shows how to opt in to a "later wins" rule with
+``Builder.allow_module_overrides()``:
+
+.. literalinclude:: ../../examples/module_overrides/module_a.py
+   :language: python
+   :lines: 19-
+   :caption: ``examples/module_overrides/module_a.py``
+
+.. literalinclude:: ../../examples/module_overrides/module_b.py
+   :language: python
+   :lines: 19-
+   :caption: ``examples/module_overrides/module_b.py``
+
+.. literalinclude:: ../../examples/module_overrides/run.py
+   :language: python
+   :lines: 18-
+   :caption: ``examples/module_overrides/run.py``
+
+When ``allow_module_overrides()`` is set, the function from the
+**later-imported** module wins, so the example above prints
+``"This is module b."``.
+
+When to reach for this pattern:
+
+* You have a stable dataflow but want a small, well-named seam for swapping
+  in a test double, a mock data source, or an environment-specific function.
+* You want the swap to be visible in the Driver-construction code, rather
+  than buried inside a function or a config flag.
+
+When *not* to reach for this pattern:
+
+* If many functions need to vary, prefer keeping the variations in distinct
+  modules and choosing which one to import. Module overrides are best as a
+  surgical tool.
+
+
+3. ``@subdag`` -- repeat the same transform inside one Driver
+-------------------------------------------------------------
+
+Sometimes you want the *same* transformation graph evaluated several times
+*inside the same DAG*, each time with a different input or configuration --
+for example, computing unique-user counts at daily / weekly / monthly grains
+across two regions.
+
+The ``@subdag`` decorator from ``hamilton.function_modifiers`` does this
+declaratively. From the source documentation:
+
+    ``@subdag`` enables you to rerun components of your DAG with varying
+    parameters. That is, it enables you to "chain" what you could express
+    with a Driver into a single DAG.
+
+The
+`reusing_functions <https://github.com/apache/hamilton/tree/main/examples/reusing_functions>`_
+example computes ``unique_users`` for two regions and three time grains.
+The shared logic lives in ``unique_users.py``:
+
+.. literalinclude:: ../../examples/reusing_functions/unique_users.py
+   :language: python
+   :lines: 18-
+   :caption: ``examples/reusing_functions/unique_users.py``
+
+Then in ``reusable_subdags.py``, each ``@subdag`` declaration creates one
+named instance of that subgraph, with its own ``inputs`` and ``config``:
+
+.. literalinclude:: ../../examples/reusing_functions/reusable_subdags.py
+   :language: python
+   :pyobject: daily_unique_users_US
+   :caption: One @subdag invocation from ``examples/reusing_functions/reusable_subdags.py``
+
+Each decorated function:
+
+* Takes the *output* of its sub-DAG as its argument. Above, the sub-DAG ends
+  in ``unique_users``, so the wrapping function receives ``unique_users:
+  pd.Series`` and returns it (perhaps after post-processing).
+* Receives ``inputs={"grain": value("day")}`` -- this binds the sub-DAG
+  input ``grain`` to the literal ``"day"`` for *this instance only*.
+* Receives ``config={"region": "US"}`` -- this scopes Hamilton's
+  ``@config.when`` selection to ``"US"`` for this sub-DAG.
+
+The same module then defines five more analogous functions (``weekly_*``,
+``monthly_*``, the ``CA`` variants), giving twelve nodes that all reuse the
+same underlying definitions.
+
+Two parameters worth knowing:
+
+* ``namespace`` -- a string prefix for the nodes that ``@subdag`` materialises.
+  By default Hamilton uses the wrapping function's name, which is normally
+  what you want.
+* ``external_inputs`` -- declare any function parameter that comes from
+  *outside* the sub-DAG (e.g. from the surrounding DAG). This makes the
+  boundary between the sub-DAG and its surroundings explicit.
+
+When to reach for this pattern:
+
+* You want one Driver, one visualised DAG, and one ``execute`` call to
+  produce all the variants -- rather than a Python ``for`` loop over many
+  Drivers in your application code.
+* You want lineage and execution metadata for every variant captured by
+  Hamilton, not by a wrapper script.
+
+
+4. ``@parameterized_subdag`` -- many subdags at once (advanced)
+---------------------------------------------------------------
+
+If you have *many* subdags that differ only along a small number of
+parameters, writing one ``@subdag`` declaration per case becomes verbose.
+``@parameterized_subdag`` is syntactic sugar that produces several subdags
+from a single decorator -- analogous to how ``@parameterize`` produces
+several nodes from one function.
+
+From the
+`source documentation <https://github.com/apache/hamilton/blob/main/hamilton/function_modifiers/recursive.py>`_:
+
+.. code-block:: python
+
+    @parameterized_subdag(
+        feature_modules,
+        from_datasource_1={"inputs": {"data": value("datasource_1.csv")}},
+        from_datasource_2={"inputs": {"data": value("datasource_2.csv")}},
+        from_datasource_3={
+            "inputs": {"data": value("datasource_3.csv")},
+            "config": {"filter": "only_even_client_ids"},
+        },
+    )
+    def feature_engineering(feature_df: pd.DataFrame) -> pd.DataFrame:
+        return feature_df
+
+Each entry below the decorator becomes one subdag, all built from the same
+``feature_modules`` but with different inputs / config.
+
+The Hamilton source itself includes a deliberate warning on this decorator:
+
+    Think about whether this feature is really the one you want -- often
+    times, verbose, static DAGs are far more readable than very concise,
+    highly parameterized DAGs.
+
+In practice: prefer the explicit form from section 3 until the repetition
+genuinely hurts. Reach for ``@parameterized_subdag`` when the parameter
+list comes from elsewhere (e.g. a config file resolved with ``@resolve``)
+or when you have a dozen-plus near-identical subdags.
+
+The full reference for both decorators lives at:
+
+* :doc:`/reference/decorators/subdag`
+* :doc:`/reference/decorators/parameterize_subdag`
+
+
+Choosing between the four patterns
+----------------------------------
+
+A short decision tree:
+
+* **The data varies, the code does not** → just build another Driver from
+  the same module (section 1).
+* **One or two named functions need to be swapped** → put the swaps in
+  another module and use ``allow_module_overrides()`` (section 2).
+* **You want N copies of the same transform graph in one DAG** → use
+  ``@subdag`` (section 3).
+* **You have many copies and the parameter list is itself data** → consider
+  ``@parameterized_subdag`` (section 4).
+
+In practice, most production Hamilton projects rely heavily on (1), use (2)
+sparingly for testing seams, reach for (3) when modeling per-segment or
+per-grain pipelines, and treat (4) as an advanced tool.
+
+
+Where to go from here
+---------------------
+
+* Walk through the runnable examples linked above:
+  `feature_engineering_multiple_contexts <https://github.com/apache/hamilton/tree/main/examples/feature_engineering/feature_engineering_multiple_contexts>`_,
+  `module_overrides <https://github.com/apache/hamilton/tree/main/examples/module_overrides>`_,
+  and
+  `reusing_functions <https://github.com/apache/hamilton/tree/main/examples/reusing_functions>`_.
+* Read :doc:`/concepts/best-practices/code-organization` for the module
+  layout that makes these patterns natural.
+* For an end-to-end deep-dive on subdags and reuse, see the
+  `Hamilton March 2024 Meetup tutorial notebook <https://github.com/DAGWorks-Inc/hamilton-tutorials/blob/main/2024-03-19/march-meetup.ipynb>`_.