Repository: explodinggradients/ragas
Issue: Synthetic Dataset Generation fail due to "ValueError: 'a' cannot be empty unless no samples are taken"
Root cause: Validation Gap

Hi, I am trying to use `TestsetGenerator` to produce a synthetic dataset paired with `LlamaIndex` and 'Ollama', it successfully completes the embedding process, but before startin the generation process the `ValueError: 'a' cannot be empty unless no samples are taken` exception is raise; I personally think this is because, for some reason, TestsetGenerator produces the double of the required embeddings for the LlamaIndex `Document`, at least this is what I can get by looking at the embeddings progress bar.

Code:
```
import json
from pathlib import Path

from ragas.testset.generator import TestsetGenerator
from ragas.testset.evolutions import simple, reasoning, multi_context
from llama_index.core import Document
from llama_index.llms.ollama import Ollama
from llama_index.embeddings.ollama import OllamaEmbedding

from knowlege.collections import Document as MyDocument 
from knowlege.chunker import chunk # tried chunking to lower the documents length

files_gen = Path("./data/").rglob("*")
files = [f.resolve() for f in files_gen]
json_files = [f for f in files if f.suffix.lower() == ".json"]

docs = []
with open(json_files[4], 'r', encoding='utf-8') as fp: 
    data = json.load(fp)
    for item in data:
        chunks = chunk(MyDocument(
            name=item['title'], 
            content=item['content']
        ))
        for c in chunks:
            docs.append(Document(text=c))   
            
generator_llm = Ollama(model='gemma:2b')
critic_llm = Ollama(model='gemma:2b')
embeddings = OllamaEmbedding(model_name='gemma:2b')
generator = TestsetGenerator.from_llama_index(
    generator_llm,
    critic_llm,
    embeddings
)

# tried with original documents
test_docs = docs[:5]
for d in test_docs:
    print(d.doc_id)
    print(len(d.text))
    
# e861670f-3439-405d-a287-a90dfc885f9e
# 219
# a44e3cc3-e498-4662-b3e7-52220145da05
# 227
# d21bd3e2-9d7d-41de-a99e-2acd538b46d3
# 274
# c1555996-fd90-436e-ab93-dbed31471551
# 596
# 7d8145b9-7d67-4d4d-a363-695e7b0f0f79
# 646

# tried with proof-of-concept documents
damn = [
    Document(text='Sensitive Data Exposure, which is more of a broad symptom rather than a root cause, the focus is on failures related to cryptography (or lack thereof). Which often lead to exposure of sensitive data.'),
    Document(text='For example, passwords, credit card numbers, health records, personal information, and business secrets require extra protection, mainly if that data falls under privacy laws'),
    Document(text='Secure software requires a secure development lifecycle, some form of secure design pattern, paved road methodology, secured component library, tooling, and threat modeling.')
]

test_set = generator.generate_with_llamaindex_docs(
    damn,
    test_size=1, 
    distributions={simple: 0.5, reasoning: 0.25, multi_context: 0.25},
    raise_exceptions=False
) 
```        

Output:
```
Filename and doc_id are the same for all nodes.
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[9], line 1
----> 1 test_set = generator.generate_with_llamaindex_docs(
      2     damn,
      3     test_size=1, 
      4     distributions={simple: 0.5, reasoning: 0.25, multi_context: 0.25},
      5     raise_exceptions=False
      6 ) 

File D:\Desktop\UNI\Tirocinio-Tesi\project\prototypes\.venv\Lib\site-packages\ragas\testset\generator.py:183, in TestsetGenerator.generate_with_llamaindex_docs(self, documents, test_size, distributions, with_debugging_logs, is_async, raise_exceptions, run_config)
    178 # chunk documents and add to docstore
    179 self.docstore.add_documents(
    180     [Document.from_llamaindex_document(doc) for doc in documents]
    181 )
--> 183 return self.generate(
    184     test_size=test_size,
    185     distributions=distributions,
    186     with_debugging_logs=with_debugging_logs,
    187     is_async=is_async,
    188     run_config=run_config,
    189     raise_exceptions=raise_exceptions,
    190 )

File D:\Desktop\UNI\Tirocinio-Tesi\project\prototypes\.venv\Lib\site-packages\ragas\testset\generator.py:279, in TestsetGenerator.generate(self, test_size, distributions, with_debugging_logs, is_async, raise_exceptions, run_config)
    268     patch_logger("ragas.llms.prompt", logging.DEBUG)
    270 exec = Executor(
    271     desc="Generating",
    272     keep_progress_bar=True,
    273     raise_exceptions=raise_exceptions,
    274     run_config=run_config,
    275 )
    277 current_nodes = [
    278     CurrentNodes(root_node=n, nodes=[n])
--> 279     for n in self.docstore.get_random_nodes(k=test_size)
    280 ]
    281 total_evolutions = 0
    282 for evolution, probability in distributions.items():

File D:\Desktop\UNI\Tirocinio-Tesi\project\prototypes\.venv\Lib\site-packages\ragas\testset\docstore.py:328, in InMemoryDocumentStore.get_random_nodes(self, k, alpha)
    325 prob = np.array(scores) * np.array(similarity_scores)
    326 prob = prob / np.sum(prob)
--> 328 nodes = rng.choice(np.array(self.nodes), size=k, p=prob).tolist()
    330 for node in nodes:
    331     idx = self.nodes.index(node)

File numpy\\random\\_generator.pyx:803, in numpy.random._generator.Generator.choice()

ValueError: a cannot be empty unless no samples are taken
```

any update?

Closing as resolved.

If you face it still in current version, feel free to reopen or open a new issue.