Make torch an optional dependency / extra

Modern packages use optional dependecies / extras for specific functionality. I assume this is very well possible for the sagemaker sdk.

A similar request was posted in discussions: https://github.com/aws/sagemaker-python-sdk/discussions/5441

I couldn't find any actual "issue" and hope this gets more visibility.

The main and most urgent topic is the dependency of pytorch which requires 354/620mb size in a clean venv. I would assume that most use cases don't need pytorch at all.

Issue 中没有指定文件或测试。首先定位 SDK 的依赖声明和安装/extras 配置，然后追踪 PyTorch 在哪里被导入或要求；将一次干净安装与建议的 extra 进行比较。完成的标准是：基础包可以在不安装 PyTorch 的情况下安装，同时依赖 PyTorch 的 SageMaker 功能仍可通过显式 extra 使用。

由索引模型根据 Issue 内容生成。