Add SGLang serving backend support to ModelDeployment

[devin-ai-integration]

Adds SGLang as an alternative serving backend alongside vLLM for ModelConfig.serve().

Changes

modal_training_gym/common/serve_sglang.py (new)

• build_sglang_serve_app() mirrors build_vllm_serve_app() but launches sglang.launch_server inside an SGLang Docker image
• Accepts the same volume / GPU / scaling parameters as the vLLM builder
• Configurable via sglang_image_tag (defaults to lmsysorg/sglang:v0.4.7-cu124) and extra_sglang_args

modal_training_gym/common/deploy.py

• DeployConfig gains backend: Literal["vllm", "sglang"] (default "vllm" — fully backward-compatible)
• New fields: extra_sglang_args, sglang_image_tag

modal_training_gym/common/models/base.py

• ModelConfig.serve() dispatches to the correct builder based on deploy.backend

Usage (Qwen3-4B example)

from modal_training_gym.common.models import Qwen3_4B, DeployConfig

model = Qwen3_4B()
model.deploy = DeployConfig(
    backend="sglang",
    gpu="H100",
    n_gpu=1,
    extra_sglang_args=["--context-length", "8192", "--mem-fraction-static", "0.80"],
)
deployment = model.serve()
print(deployment.generate("Hello!"))

Checklist

• Example is documented with comments throughout, in a Literate Programming style.
• Example does not require third-party dependencies to be installed locally
• Example pins its dependencies
  • Example pins container images to a stable tag, not a dynamic tag like latest
  • Example specifies a python_version for the base image, if it is used
  • Example pins all dependencies to at least minor version, ~=x.y.z or ==x.y
  • Example dependencies with version < 1 are pinned to patch version, ==0.y.z

Outside contributors

N/A — internal change.

Link to Devin session: https://app.devin.ai/sessions/6aac27bf99024337a1d27328aac58062
Requested by: @joyliu-q

[devin-ai-integration]

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