Extract generation_manager from tokenizer_manager

[fzyzcjy]

Motivation

TODO: briefly explain

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.

[fzyzcjy]

All these PRs are waiting for CI (p.s. I have run test_fragment and torchrun script on non-CI and it works)