Repository: explodinggradients/ragas
Issue: Haystack evaluation with ragas not working for googlegenai models
Root cause: Unimplemented Feature Gap

[x ] I have checked the [documentation](https://docs.ragas.io/) and related resources and couldn't resolve my bug.

**Describe the bug**

UPDATE: It looks like it is not possible to use Gemini with ragas and Haystack, so I tried to implement a custom BaseRagasLLM class and a custom HuggingfaceEmbedding class myself, but then more and more mismatches are hapenning between ragas and Haystack that are tedius to implement fixes for. I guess then this is more of a feature request, so I will close the issue because it is not a bug per say (even though the error message of not having haystack-ai installed is incorrect).

Original bug description:
I am using a Haystack RAG pipeline and I want to extend the pipeline with an evaluation step with ragas. I am following this tutorial: https://haystack.deepset.ai/integrations/ragas. 
My issue is that I think the Gemini models are not supported by default and I am also unsure how to work around it.
When I create a `RagasEvaluator` (defined as in ` haystack_integrations.components.evaluators.ragas.RagasEvaluator`), it expects an evaluator_llm parameter of type `ragas.llms.HaystackLLMWrapper`. It seems `HaystackLLMWrapper` can only be models of the folllowing types: `AzureOpenAIGenerator | HuggingFaceAPIGenerator | HuggingFaceLocalGenerator | OpenAIGenerator `. I am using `haystack_integrations.components.generators.google_genai.GoogleGenAIChatGenerator` to generate answers. 
I get an import error: `ImportError: cannot import name 'GoogleGenAIChatGenerator' from 'haystack.components.generators'`

Ragas version: 0.2.15
Python version: 3.10

**Code to Reproduce**

```
from haystack_integrations.components.generators.google_genai import GoogleGenAIChatGenerator
from haystack_integrations.components.evaluators.ragas import RagasEvaluator
from ragas.llms import HaystackLLMWrapper
from ragas.metrics import AnswerRelevancy, ContextPrecision, Faithfulness

llm = GoogleGenAIChatGenerator()
evaluator_llm = HaystackLLMWrapper(llm)

ragas_evaluator = RagasEvaluator(
    ragas_metrics=[AnswerRelevancy(), ContextPrecision(), Faithfulness()],
    evaluator_llm=evaluator_llm,
)
```

**Error trace**

```
Traceback (most recent call last):
  File "/home/konstantina/anaconda3/envs/rag_env/lib/python3.10/site-packages/ragas/llms/haystack_wrapper.py", line 43, in __init__
    from haystack.components.generators import (
ImportError: cannot import name 'GoogleGenAIChatGenerator' from 'haystack.components.generators' (/home/konstantina/anaconda3/envs/rag_env/lib/python3.10/site-packages/haystack/components/generators/__init__.py)

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/konstantina/projects/faq_chatbot/faq_rag_pipeline.py", line 115, in evaluate_rag_pipeline
    evaluator = HaystackLLMWrapper(chat_generator)
  File "/home/konstantina/anaconda3/envs/rag_env/lib/python3.10/site-packages/ragas/llms/haystack_wrapper.py", line 51, in __init__
    raise ImportError(
ImportError: Haystack is not installed. Please install it using `pip install haystack-ai`.

```

**Expected behavior**
This `evaluator = HaystackLLMWrapper(chat_generator)` to work with `GoogleGenAIChatGenerator` as chat_generator.

**Additional context**
Add any other context about the problem here.

<!-- PS: bugs suck but is also part of the process. We sincerely apologies for breaking your flow because of it, but don't worry, we got your back ❤️. We will get this fixed as fast as we can and thanks for helping us out by reporting it 🙏. -->