[WASI-NN] Add support for a PyTorch backend for wasi-nn

[rahulchaphalkar]

This change adds a PyTorch backend for wasi-nn. tch crate is used for Libtorch bindings. I have added an image classification example to demonstrate its usage, which uses a torchscript model. This backend is currently gated behind a wasi-nn feature flag --features pytorch as due to dynamic linking, a Libtorch v2.4.0 installation on the system (specified by LIBTORCH=/path/to/libtorch) is needed for building.