[rewriter] Decouple llama rule sets

[titaiwangms]

https://github.com/microsoft/onnxscript/blob/main/onnxscript/rewriter/llama_rule_sets.py

The file contains a lot of rewrite rules, but it's name is misleading, and its test (https://github.com/microsoft/onnxscript/blob/main/onnxscript/rewriter/llama_rule_sets_test.py) is also tangled. We need the API in optimizer explicitly specify what rules we are applying.

onnxscript/onnxscript/optimizer/_optimizer.py

Line 20 in cde945d

_DEFAULT_REWRITE_RULES = [