Update on "add option to run mmlu with 5 shots"

This PR does the following changes:
- add `--num_fewshot` option which is required for running MMLU task with 5 shots
- set the default value of `--limit` to none such that we can actually run all examples
- update `eval_llama` to call `simple_evaluate` which is a wrapper of `evaluate` and does some extra work for us like getting the task dict

Test Plan:
- Make sure WikiText perplexity for llama 3.2 1B stays the same before and after the change.

Before, run eval_llama for llama 3.2 1B with limit set to None:
```
wikitext: {'word_perplexity,none': 12.78246428138387, 'word_perplexity_stderr,none': 'N/A', 'byte_perplexity,none': 1.610432252171856, 'byte_perplexity_stderr,none': 'N/A', 'bits_per_byte,none': 0.6874479705552373, 'bits_per_byte_stderr,none': 'N/A', 'alias': 'wikitext'}
```
After, run eval_llama for llama 3.2 1B:
```
wikitext: {'word_perplexity,none': 12.78246428138387, 'word_perplexity_stderr,none': 'N/A', 'byte_perplexity,none': 1.610432252171856, 'byte_perplexity_stderr,none': 'N/A', 'bits_per_byte,none': 0.6874479705552373, 'bits_per_byte_stderr,none': 'N/A', 'alias': 'wikitext'}
```
- Make sure that lm_eval(v0.4.2, which is used by eval_llama) and eval_llama reports similar number for llama 3.2 1B and 3B BF16 for MMLU task with 5 shots.

Example command for lm_eval:
```
lm_eval --model hf \
   --model_args pretrained=meta-llama/Llama-3.2-1B-Instruct \
   --tasks mmlu \
   --device cuda \
   -f 5 \
   --batch_size auto
```
Example command for eval_llama:
```
python -m examples.models.llama2.eval_llama \
	-c /home/lunwenh/models/1B_Instruct/consolidated.00.pth \
	-p /home/lunwenh/models/1B_Instruct/params.json \
	-t /home/lunwenh/models/1B_Instruct/tokenizer.model \
	-kv \
	-d bf16 \
	--tasks mmlu \
        -f 5 \
	--max_seq_length 2048
```

Differential Revision: [D64215268](https://our.internmc.facebook.com/intern/diff/D64215268)

[ghstack-poisoned]

Author: helunwencser

examples/models/llama2/eval_llama_lib.py

@@ -247,7 +247,10 @@ def build_args_parser() -> argparse.ArgumentParser:
         help="list of lm-eluther tasks to evaluate usage: --tasks task1 task2",
     )
     parser.add_argument(
-        "--limit", type=int, default=None, help="number of samples to evalulate"
+        "--limit",
+        type=int,
+        default=None,
+        help="number of samples to evalulate. If not set, evaluate all samples",
     )
     parser.add_argument(
         "-f",