fix(testset): for generating testset with new docs (#1001)

fixes #848 

This pull request introduces updates to the TestsetGenerator and
ComplexEvolution classes within the generator.py and evolutions.py files
respectively. The primary focus of these changes is to improve the
handling of new documents during test dataset generation, ensuring that
data from previous documents does not interfere with the generation
process.

### Before
![Screenshot 2024-05-31 183725
before](https://github.com/explodinggradients/ragas/assets/77217074/dcc24e81-2e35-4e20-8638-2904d6779225)
Here row 2 and 4 are generated from the document used for the previous
generation
### After
![Screenshot 2024-05-31 184647
after](https://github.com/explodinggradients/ragas/assets/77217074/582afeeb-9ade-4d35-891c-af54af21c859)
Here all rows generated are based on the new document provided

src/ragas/testset/evolutions.py

@@ -352,14 +352,13 @@ def init(self, is_async: bool = True, run_config: t.Optional[RunConfig] = None):
             run_config = RunConfig()
         super().init(is_async=is_async, run_config=run_config)
 
-        if self.se is None:
-            # init simple evolution to get seed question
-            self.se = SimpleEvolution(
-                generator_llm=self.generator_llm,
-                docstore=self.docstore,
-                node_filter=self.node_filter,
-                question_filter=self.question_filter,
-            )
+        # init simple evolution to get seed question
+        self.se = SimpleEvolution(
+            generator_llm=self.generator_llm,
+            docstore=self.docstore,
+            node_filter=self.node_filter,
+            question_filter=self.question_filter,
+        )
         # init evolution filter with critic llm from another filter
         assert self.node_filter is not None, "node filter cannot be None"
         if self.evolution_filter is None:

src/ragas/testset/generator.py

@@ -217,11 +217,11 @@ def generate_with_langchain_docs(
         )
 
     def init_evolution(self, evolution: Evolution) -> None:
+        evolution.docstore = self.docstore
+
         if evolution.generator_llm is None:
             evolution.generator_llm = self.generator_llm
-            if evolution.docstore is None:
-                evolution.docstore = self.docstore
-
+
             if evolution.question_filter is None:
                 evolution.question_filter = QuestionFilter(llm=self.critic_llm)
             if evolution.node_filter is None: