Extract generation_manager from tokenizer_manager

[fzyzcjy]

Motivation

A large portion of TokenizerManager (or indeed Orchestrator, as is renamed in #3116) is related to generation, and the generation logic is somehow isolated from the other parts. Thus this is extracted.

Moreover, by doing this refactor, adding SPMD logic would be a bit easier and cleaner.

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)

[ch-wan]

Thanks for the elaboration and efforts in addressing my questions. This PR LGTM now.