docs: add a document of how to create InMemoryTarget

Author: utotsubasa

docs/task_on_kart.rst

@@ -286,3 +286,24 @@ If you want to dump csv file with other encodings, you can use `encoding` parame
     def output(self):
         return self.make_target('file_name.csv', processor=CsvFileProcessor(encoding='cp932'))
         # This will dump csv as 'cp932' which is used in Windows.
+
+Cache output in memory instead of dumping to files
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+You can use :class:`~InMemoryTarget` to cache output in memory instead of dumping to files by calling :func:`~gokart.target.make_inmemory_target`.
+
+.. code:: python
+
+    from gokart.in_memory.target import make_inmemory_target
+
+    def output(self):
+    unique_id = self.make_unique_id() if use_unique_id else None
+    # TaskLock is not supported in InMemoryTarget, so it's dummy
+    task_lock_params = make_task_lock_params(
+        file_path='dummy_path',
+        unique_id=unique_id,
+        redis_host=None,
+        redis_port=None,
+        redis_timeout=self.redis_timeout,
+        raise_task_lock_exception_on_collision=False,
+    )
+    return make_inmemory_target('dummy_path', task_lock_params, unique_id)