Add num_generations and group_generations parameters to Task

[gabrielmbmb]

Description

This PR adds the num_generations runtime parameter which will be used to define the number of generations to create per input using an LLM. In addition, the group_generations parameter is added which defines whether a list with the num_generations is created or if a row with each generation is created:

# num_generations = 3, group_generations = True
[
    {"instruction": "Hello, how are you?", "generation": ["Hi, I'm fine", "Hi, I'm fine, thank you for asking", "Hi! I'm fine and you?"]}
]

# num_generations = 3, group_generations = False
[
    {"instruction": "Hello, how are you?", "generation": "Hi, I'm fine"},
    {"instruction": "Hello, how are you?", "generation": "Hi, I'm fine, thank you for asking"},
    {"instruction": "Hello, how are you?", "generation": "Hi! I'm fine and you?"},
]

In addition, this PR has updated all the LLMs implementations to handle the num_generations parameter.

Closes #413

[alvarobartt]

LGTM, thanks! Just some comments to tackle before approving and merging!