Add more tests about SPMD

[fzyzcjy]

Motivation

The tests can be run locally by executing test_fragment.py. (It is skipped in CI, which mimics the test_generation_models.py).

Modifications

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
• For reviewers: If you haven't made any contributions to this PR and are only assisting with merging the main branch, please remove yourself as a co-author when merging the PR.
• Please feel free to join our Slack channel at https://slack.sglang.ai to discuss your PR.

[fzyzcjy]

After discussing with Lianmin, now I know there is no need to follow implementation requirements in e.g. 2736. Thus I spent several hours writing a new PR #3852, and this one is deprecated.