Fix testset generator issue on context selection (#937)

This pull request addresses an issue in the TestsetGenerator where the
chosen contexts for evolutions were not being sampled correctly because
only the very first contexts were selected. Now, the selection can
potentially choose any context

Co-authored-by: Jules Decaestecker <[email protected]>

Author: Jovillios

src/ragas/testset/generator.py

@@ -3,7 +3,7 @@
 import logging
 import typing as t
 from dataclasses import dataclass
-from random import choices
+from random import choices, sample
 
 import pandas as pd
 from datasets import Dataset
@@ -242,7 +242,7 @@ def generate(
         ]
         total_evolutions = 0
         for evolution, probability in distributions.items():
-            for i in range(round(probability * test_size)):
+            for i in sample(range(test_size),round(probability * test_size)):
                 exec.submit(
                     evolution.evolve,
                     current_nodes[i],