Evals: Fix Fewshots

[pefontana]

We need to do a double check in our evalutions with fewshots.

1. We are tokenizing the fewshots and the question separatly and the extendinf the String
   https://github.com/PsycheFoundation/psyche/blob/main/shared/eval/src/harness.rs#L135
   https://github.com/PsycheFoundation/psyche/blob/main/shared/eval/src/harness.rs#L148

That can lead to some mismatch in the tokens generations
For example in our implementation of arc easy and arc challenge, if the eval has fewshots, if we decode the eval request we have and additional space
' Answer: ....'

You can see the Hugging Face request to compare here batched_inps:
https://github.com/EleutherAI/lm-evaluation-harness/blob/cd9bac7c27f3c876bb8e60dca8ee3b6de6b33c35/lm_eval/models/huggingface.py#L1296-L1297

2. I think here we need an extra space
   

   psyche/shared/eval/src/harness.rs

   Lines 243 to 244 in abb4335

   	.map(|x| format!("{}{}", x.text, x.choices[x.answer]))
   	.collect::<Vec<_>>()

We should check all the evals and see that our implementation has the same format as lm_eval