Set IPEX_LLM_LAST_LM_HEAD=1 as default

[cyita]

Description

1. Set IPEX_LLM_LAST_LM_HEAD=1 as default. The user can export IPEX_LLM_LAST_LM_HEAD=0 to disable this optimization.
2. Add export IPEX_LLM_LAST_LM_HEAD=0 description in the accuracy test.

1. Why the change?

2. User API changes

3. Summary of the change

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.
• Application test
• Document test
• ...

5. New dependencies

• New Python dependencies
  - Dependency1
  - Dependency2
  - ...
• New Java/Scala dependencies and their license
  - Dependency1 and license1
  - Dependency2 and license2
  - ...

[jason-dai]

LGTM

[cyita]

PR Validation: https://github.com/intel-analytics/ipex-llm-workflow/actions/runs/10484590187

[rnwang04]

LGTM