Fix reporting backends and dtyep to benchmark results (#6023)

Summary:
Couple minor fixes for reporting the benchmarking results:
 - qnn models are not reporting "backend" and "dtype" info in the benchmark_results.json (Android)
 - tinyllama mdoel is not reporting "backend" and "dtype" info in the benchmark_results.json (Android)
 - include compute precision to the exported coreml model name
 - rename "llama2" to "tinyllama" to eliminate confusion (many people thought it was llama2-7b)

Pull Request resolved: https://github.com/pytorch/executorch/pull/6023

Reviewed By: huydhn

Differential Revision: D64074262

Pulled By: guangy10

fbshipit-source-id: c6c53d004c4fb3ad410a792639af2c22a6978b67