Update on "[slimtensor] Add from_etensor factory function for ETensor to SlimTensor conversion"

Add from_etensor() factory function that creates a SlimTensor from an ExecuTorch portable tensor (ETensor), copying data to a target device.

Key features:
- Handles int32_t to int64_t conversion for sizes/strides (ETensor uses int32_t, SlimTensor uses int64_t)
- Supports CPU and CUDA target devices via storage()->copy_()
- Preserves tensor strides (non-contiguous layouts)
- Provides both reference and pointer overloads

Differential Revision: [D90539554](https://our.internmc.facebook.com/intern/diff/D90539554/)

[ghstack-poisoned]

Author: Gasoonjia