Add checking valid inputs before calling _generate

[gabrielmbmb]

Description

This PR updates the generate method of the LLM class to:

1. Check which inputs are valid (have the required task.input_args_names keys) getting a list of the valid ones and a list containing the indices of the invalid ones.
2. Call _generate with the list of valid inputs
3. Fill the indices of the invalid inputs of the final list with empty LLMOutputs, as the LLM could not be executed for these inputs.

Close #208