[NPU] Support non-const parameter for decoder layers when keep_ir=True

[rnwang04]

Description

1. Why the change?

• Work with https://github.com/intel-analytics/llm.cpp/pull/829 to support non-const layernorm & bias

2. User API changes

• For decoder layers, we default make layernorm & bias as const, but for now, we can make them as input by passing keep_ir=True or set IPEX_LLM_NPU_CONST_PARAMETER=0

3. Summary of the change

• Update parameter name from layernorm_const to const_parameter
• Set const_parameter to False when passing keep_ir=True or set IPEX_LLM_NPU_CONST_PARAMETER=0
• Make layernorm & bias as input when const_parameter is True

4. How to test?

• N/A
• Unit test: Please manually trigger the PR Validation here by inputting the PR number (e.g., 1234). And paste your action link here once it has been successfully finished.