[feat] Add option to mask prompts with left-padded tokenizer and corpus and query prompts to IREvaluator

[ArthurCamara]

Most models, specially based on Mistral, are trained with left-padding in the tokenizer, instead of the common right-padding when evaluating. (e.g., LLM2Vec), but this is not covered in the current prompt_length strategy.
Currently, If a batch with mismatched lengths is left-padded, the masking will mostly mask out padding tokens. This PR fixes that by adding a mask_prompt argument to the tokenize function. When this flag is set, the code tries to find the first non-padding token in each sentence, and will mask everything between that and prompt_length, adding a prompt_mask representation to the output dictionary.

Perhaps a more "elegant" solution would be to replace prompt_length entirely, but this could break Instructor models.