Add scaffold for defining custom components for Pipelines (#1205)

rest_api/config.py

@@ -1,6 +1,6 @@
 import os
 
-PIPELINE_YAML_PATH = os.getenv("PIPELINE_YAML_PATH", "rest_api/pipelines.yaml")
+PIPELINE_YAML_PATH = os.getenv("PIPELINE_YAML_PATH", "rest_api/pipeline/pipelines.yaml")
 QUERY_PIPELINE_NAME = os.getenv("QUERY_PIPELINE_NAME", "query")
 INDEXING_PIPELINE_NAME = os.getenv("INDEXING_PIPELINE_NAME", "indexing")
 

rest_api/controller/__init__.py

@@ -0,0 +1 @@
+from rest_api.pipeline import custom_component  # this import is required for the Custom Components to be registered

rest_api/pipeline/custom_component.py

@@ -0,0 +1,17 @@
+"""
+Pipelines allow putting together Components to build a graph.
+
+In addition to the standard Haystack Components, custom user-defined Components
+can be used in a Pipeline YAML configuration.
+
+The classes for the Custom Components must be defined in this file.
+"""
+
+
+from haystack import BaseComponent
+
+
+class SampleComponent(BaseComponent):
+
+    def run(self, **kwargs):
+        raise NotImplementedError