fix TimeoutError in aiohttp

[casper-hansen]

Checklist Before Starting

• Search for similar PR(s).

What does this PR do?

fixes TimeoutError that #1588 reintroduces. see #1588 (comment)

High-Level Design

Demonstrate the high-level design if this PR is complex.

Specific Changes

List the specific changes.

• add aiohttp.Timeout to ClientSession for unlimited generation time

API

Demonstrate how the API changes if any.

Usage Example

Provide usage example(s) for easier usage.

# Add code snippet or script demonstrating how to use this 

Test

For changes that can not be tested by CI (e.g., algorithm implementation, new model support), validate by experiment(s) and show results like training curve plots, evaluatuion results, etc.

Additional Info.

• Issue Number: Fixes issue # or discussion # if any.
• Training: [Note which backend this PR will affect: FSDP, Megatron, both, or none]
• Inference: [Note which backend this PR will affect: vLLM, SGLang, both, or none]

Checklist Before Submitting

• Read the Contribute Guide.
• Apply pre-commit checks.
• Add [BREAKING] to the PR title if it breaks any API.
• Update the documentation about your changes in the docs.
• Add CI test(s) if necessary.