issues with llama_index dependency

[ceferisbarov]

Describe the bug
llama_index is imported in multiple places without any exception handling, but it is not added to the requirements. I appreciate the idea of avoiding bloated requirements, but we should probably add a user-friendly error message. You actually have done this in some places with the unstructured library.

Ragas version: 0.0.20
Python version: 3.10.12

Code to Reproduce

from langchain.document_loaders import DirectoryLoader
from ragas.testset import TestsetGenerator

loader = DirectoryLoader("./data", glob="*.txt")
documents = loader.load()

testsetgenerator = TestsetGenerator.from_default()
test_size = 10
testset = testsetgenerator.generate(documents, test_size=test_size)

Error trace

Traceback (most recent call last):
  File "/home/jafar/projects/rag-evaluate/src/generate_testset.py", line 2, in <module>
    from ragas.testset import TestsetGenerator
  File "/home/jafar/projects/rag-evaluate/env/lib/python3.10/site-packages/ragas/testset/__init__.py", line 1, in <module>
    from ragas.testset.testset_generator import TestsetGenerator
  File "/home/jafar/projects/rag-evaluate/env/lib/python3.10/site-packages/ragas/testset/testset_generator.py", line 16, in <module>
    from llama_index.node_parser.simple import SimpleNodeParser
ModuleNotFoundError: No module named 'llama_index.node_parser.simple'

Expected behavior
The following line should be printed:

ValueError: llama_index package not found, please install it with `pip install llama_index`

Additional context
There is also another issue regarding llama_index versions. I believe that's also relevant to the discussion, so I will create a new issue and attach here.

[ceferisbarov]

Now that the PR has been merged, I guess we can close this.