Add eval for eager et (#2856)

Summary: Pull Request resolved: https://github.com/pytorch/executorch/pull/2856

Test Plan:
Imported from GitHub, without a `Test Plan:` line.
```
python3 -m examples.models.llama2.eval_llama --pte stories_fp32.pte -t ../llama-models/stories/tokenizer.model
```
Results:
```
100%|███████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 5/5 [2:21:53<00:00, 1702.63s/it]
wikitext: {'word_perplexity,none': 10885.21588720899, 'word_perplexity_stderr,none': 'N/A', 'byte_perplexity,none': 6.144013580095854, 'byte_perplexity_stderr,none': 'N/A', 'bits_per_byte,none': 2.619181404847365, 'bits_per_byte_stderr,none': 'N/A', 'alias': 'wikitext'}
```

Reviewed By: Jack-Khuu

Differential Revision: D55777043

Pulled By: lucylq

fbshipit-source-id: d3d52bc5fd7674d69c2bf36a084974f3276d32a7